diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..e4122ce
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,3 @@
+/Manifest.toml
+*.cov
+/lcov.info
diff --git a/LICENSE b/LICENSE
new file mode 100644
index 0000000..889ccea
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,22 @@
+MIT License
+
+Copyright (c) 2023 Fredrik Ekre and Prometheus.jl contributors
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+
diff --git a/Project.toml b/Project.toml
new file mode 100644
index 0000000..20ccf2d
--- /dev/null
+++ b/Project.toml
@@ -0,0 +1,13 @@
+name = "Prometheus"
+uuid = "f25c1797-fe98-4e0c-b252-1b4fe3b6bde6"
+version = "1.0.0"
+
+[deps]
+HTTP = "cd3eb016-35fb-5094-929b-558a96fad6f3"
+Sockets = "6462fe0b-24de-5631-8697-dd941f90decc"
+
+[extras]
+Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
+
+[targets]
+test = ["Test"]
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..dbf7e7a
--- /dev/null
+++ b/README.md
@@ -0,0 +1,90 @@
+# Prometheus.jl
+
+*A [Prometheus](https://prometheus.io/) client for Julia*
+
+## Quickstart
+
+1. Install Prometheus.jl and [HTTP.jl](https://github.com/JuliaWeb/HTTP.jl)
+   using the package manager:
+   ```
+   pkg> add Prometheus HTTP
+   ```
+
+2. Paste the following code into a Julia REPL.
+   ```julia
+   # Load the packages
+   using Prometheus, HTTP
+
+   # Create a Counter metric
+   const request_counter = Prometheus.Counter("request_count", "Number of handled requests")
+
+   # Start a HTTP server on localhost port 8000 to server the metrics
+   server = HTTP.listen!(8000) do http
+       Prometheus.inc(request_counter) # Increment the request counter
+       return Prometheus.expose(http)  # Expose the metrics
+   end
+   ```
+
+3. Visit <http://localhost:8000> in your browser. You will see something like the following
+   ```
+   # HELP request_count Number of handled requests
+   # TYPE request_count counter
+   request_count 1
+   ```
+   which is how the counter is presented when Prometheus scrapes the metrics.
+   Every time you refresh, the counter will increment its value.
+   `close(server)` will shutdown the server.
+
+
+## Collectors
+
+### Counter
+
+See <https://prometheus.io/docs/concepts/metric_types/#counter> for details.
+
+Supported methods:
+ - `Prometheus.inc(counter)`: increment the counter with 1.
+ - `Prometheus.inc(counter, v)`: increment the counter with `v`.
+
+### Gauge
+
+See <https://prometheus.io/docs/concepts/metric_types/#gauge> for details.
+
+Supported methods:
+ - `Prometheus.inc(gauge)`: increment the gauges's value with 1.
+ - `Prometheus.inc(gauge, v)`: increment the gauge's value with `v`.
+ - `Prometheus.dec(gauge)`: decrement the gauges's value with 1.
+ - `Prometheus.dec(gauge, v)`: decrement the gauge's value with `v`.
+ - `Prometheus.set_to_current_time(gauge)`: set the gauge's value to the current unixtime in
+   seconds.
+
+### Summary
+
+See <https://prometheus.io/docs/concepts/metric_types/#summary> for details.
+
+Supported methods:
+ - `Prometheus.observe(summary, v)`: record the observed value `v`.
+
+## Labels
+
+See <https://prometheus.io/docs/concepts/data_model/#metric-names-and-labels> for details.
+
+All metrics can be labeled using the special `Prometheus.Family` collector. For example, a
+labeled Counter collector
+```julia
+labelnames = ["endpoint", "status_code"]
+counter_family = Prometheus.Family{Prometheus.Collector}(
+    "http_requests",
+    "Number of processed requests",
+    labelnames,
+)
+```
+
+Supported methods:
+ - `Prometheus.labels(family, ["label 1", "label 2"])`: extract the child collector
+   corresponding to the labels `["label 1", "label 2"]`.
+ - `Prometheus.remove(family, ["label 1", "label 2"])`: remove the child collector
+   corresponding to the labels `["label 1", "label 2"]`.
+ - `Prometheus.clear(family)`: clear all child collectors.
+
+## Registries
diff --git a/src/Prometheus.jl b/src/Prometheus.jl
new file mode 100644
index 0000000..7a0ed03
--- /dev/null
+++ b/src/Prometheus.jl
@@ -0,0 +1,530 @@
+module Prometheus
+
+using HTTP: HTTP
+using Sockets: Sockets
+
+abstract type Collector end
+
+#########
+# Utils #
+#########
+
+abstract type PrometheusException <: Exception end
+
+struct PrometheusUnreachable <: PrometheusException end
+unreachable() = throw(PrometheusUnreachable())
+
+struct PrometheusAssert <: PrometheusException end
+macro assert(cond)
+    return quote
+        $(esc(cond)) || throw(PrometheusAssert())
+    end
+end
+
+###########################################
+# Compat for const fields, @lock, @atomic #
+###########################################
+@eval macro $(Symbol("const"))(field)
+    if VERSION >= v"1.8.0-DEV.1148"
+        Expr(:const, esc(field))
+    else
+        return esc(field)
+    end
+end
+if VERSION < v"1.7.0"
+    # Defined but not exported
+    using Base: @lock
+end
+if !isdefined(Base, Symbol("@atomic")) # v1.7.0
+    const ATOMIC_COMPAT_LOCK = ReentrantLock()
+    macro atomic(expr)
+        if Meta.isexpr(expr, :(::))
+            return esc(expr)
+        else
+            return quote
+                lock(ATOMIC_COMPAT_LOCK)
+                tmp = $(esc(expr))
+                unlock(ATOMIC_COMPAT_LOCK)
+                tmp
+            end
+        end
+    end
+end
+
+
+#####################
+# CollectorRegistry #
+#####################
+
+struct CollectorRegistry
+    lock::ReentrantLock
+    collectors::Base.IdSet{Collector}
+    function CollectorRegistry()
+        return new(ReentrantLock(), Base.IdSet{Collector}())
+    end
+end
+
+const DEFAULT_REGISTRY = CollectorRegistry()
+
+function register(reg::CollectorRegistry, collector::Collector)
+    existing_names = Set{String}() # TODO: Cache existing_names in the registry?
+    @lock reg.lock begin
+        for c in reg.collectors
+            union!(existing_names, metric_names(c))
+        end
+        if any(in(existing_names), metric_names(collector))
+            error("not allowed")
+        end
+        push!(reg.collectors, collector)
+    end
+    return
+end
+
+function unregister(reg::CollectorRegistry, collector::Collector)
+    @lock reg.lock delete!(reg.collectors, collector)
+    return
+end
+
+##############
+# Collectors #
+##############
+
+# abstract type Collector end
+
+function collect(collector::Collector)
+    return collect!(Metric[], collector)
+end
+
+########################
+# Counter <: Collector #
+########################
+# https://prometheus.io/docs/instrumenting/writing_clientlibs/#counter
+
+# TODO: A counter is ENCOURAGED to have:
+#  - A way to count exceptions throw/raised in a given piece of code, and optionally only
+#    certain types of exceptions. This is count_exceptions in Python.
+
+mutable struct Counter <: Collector
+    @const metric_name::String
+    @const help::String
+    @atomic value::Float64
+
+    function Counter(
+            registry::Union{CollectorRegistry, Nothing},
+            metric_name::String, help::String,
+        )
+        initial_value = 0.0
+        counter = new(metric_name, help, initial_value)
+        if registry !== nothing
+            register(registry, counter)
+        end
+        return counter
+    end
+end
+
+function Counter(metric_name::String, help::String)
+    return Counter(DEFAULT_REGISTRY, metric_name, help)
+end
+
+function metric_names(counter::Counter)
+    return (counter.metric_name, )
+end
+
+function inc(m::Counter, v = 1.0)
+    if v < 0
+        error("counting backwards")
+    end
+    @atomic m.value += v
+    return nothing
+end
+
+function collect!(metrics::Vector, counter::Counter)
+    push!(metrics,
+        Metric(
+            "counter", counter.metric_name, counter.help,
+            nothing, Sample(nothing, nothing, @atomic(counter.value)),
+        ),
+    )
+    return metrics
+end
+
+
+######################
+# Gauge <: Collector #
+######################
+# https://prometheus.io/docs/instrumenting/writing_clientlibs/#gauge
+
+# TODO: A gauge is ENCOURAGED to have:
+#  - A way to track in-progress requests in some piece of code/function. This is
+#    track_inprogress in Python.
+#  - A way to time a piece of code and set the gauge to its duration in seconds. This is
+#    useful for batch jobs. This is startTimer/setDuration in Java and the time()
+#    decorator/context manager in Python. This SHOULD match the pattern in Summary/Histogram
+#    (though set() rather than observe()).
+
+mutable struct Gauge <: Collector
+    @const metric_name::String
+    @const help::String
+    @atomic value::Float64
+
+    function Gauge(
+            registry::Union{CollectorRegistry, Nothing},
+            metric_name::String, help::String,
+        )
+        initial_value = 0.0
+        gauge = new(metric_name, help, initial_value)
+        if registry !== nothing
+            register(registry, gauge)
+        end
+        return gauge
+    end
+end
+
+function Gauge(metric_name::String, help::String)
+    return Gauge(DEFAULT_REGISTRY, metric_name, help)
+end
+
+function metric_names(gauge::Gauge)
+    return (gauge.metric_name, )
+end
+
+function inc(m::Gauge, v = 1.0)
+    if v < 0
+        error("incrementing with negative value, use dec(...)?")
+    end
+    @atomic m.value += v
+    return nothing
+end
+
+function dec(m::Gauge, v = 1.0)
+    if v < 0
+        error("decrementing with negative value, use inc(...)?")
+    end
+    @atomic m.value -= v
+    return nothing
+end
+
+function set(m::Gauge, v)
+    @atomic m.value = v
+    return nothing
+end
+
+function set_to_current_time(m::Gauge)
+    @atomic m.value = time()
+    return nothing
+end
+
+function collect!(metrics::Vector, gauge::Gauge)
+    push!(metrics,
+        Metric(
+            "gauge", gauge.metric_name, gauge.help,
+            nothing, Sample(nothing, nothing, @atomic(gauge.value)),
+        ),
+    )
+    return metrics
+end
+
+
+########################
+# Summary <: Collector #
+########################
+# https://prometheus.io/docs/instrumenting/writing_clientlibs/#summary
+
+# TODO: A summary SHOULD have the following methods:
+#  - Some way to time code for users in seconds. In Python this is the time()
+#    decorator/context manager. In Java this is startTimer/observeDuration. Units other than
+#    seconds MUST NOT be offered (if a user wants something else, they can do it by hand).
+#    This should follow the same pattern as Gauge/Histogram.
+
+mutable struct Summary <: Collector
+    @const metric_name::String
+    @const help::String
+    @atomic _count::Int
+    @atomic _sum::Float64
+
+    function Summary(
+            registry::Union{CollectorRegistry, Nothing},
+            metric_name::String, help::String,
+        )
+        initial_count = 0
+        initial_sum = 0.0
+        summary = new(metric_name, help, initial_count, initial_sum)
+        if registry !== nothing
+            register(registry, summary)
+        end
+        return summary
+    end
+end
+
+function Summary(metric_name::String, help::String)
+    return Summary(DEFAULT_REGISTRY, metric_name, help)
+end
+
+function metric_names(summary::Summary)
+    return (summary.metric_name * "_count", summary.metric_name * "_sum")
+end
+
+function observe(summary::Summary, v)
+    @atomic summary._count += 1
+    @atomic summary._sum += v
+    return nothing
+end
+
+function collect!(metrics::Vector, summary::Summary)
+    push!(metrics,
+        Metric(
+            "summary", summary.metric_name, summary.help, nothing,
+            [
+                Sample("_count", nothing, @atomic(summary._count)),
+                Sample("_sum", nothing, @atomic(summary._sum)),
+            ]
+        ),
+    )
+    return metrics
+end
+
+
+####################################
+# Family{<:Collector} <: Collector #
+####################################
+
+struct LabelNames
+    labelnames::Vector{String}
+end
+
+struct LabelValues
+    labelvalues::Vector{String}
+end
+function Base.hash(l::LabelValues, h::UInt)
+    h = hash(0x94a2d04ee9e5a55b, h) # hash("Prometheus.LabelValues") on Julia 1.9.3
+    for v in l.labelvalues
+        h = hash(v, h)
+    end
+    return h
+end
+function Base.:(==)(l1::LabelValues, l2::LabelValues)
+    return l1.labelvalues == l2.labelvalues
+end
+
+struct Family{C} <: Collector
+    metric_name::String
+    help::String
+    labelnames::LabelNames
+    children::Dict{LabelValues, C}
+    lock::ReentrantLock
+
+    function Family{C}(
+            registry::Union{CollectorRegistry, Nothing},
+            metric_name::String, help::String, labelnames::LabelNames,
+        ) where C
+        children = Dict{LabelValues, C}()
+        lock = ReentrantLock()
+        family = new(metric_name, help, labelnames, children, lock)
+        if registry !== nothing
+            register(registry, family)
+        end
+        return family
+    end
+end
+
+function Family{C}(metric_name::String, help::String, labelnames) where C
+    return Family{C}(DEFAULT_REGISTRY, metric_name, help, LabelNames(labelnames))
+end
+function Family{C}(registry::Union{CollectorRegistry, Nothing}, metric_name::String, help::String, labelnames) where C
+    return Family{C}(registry, metric_name, help, LabelNames(labelnames))
+end
+
+function metric_names(family::Family)
+    return (family.metric_name, )
+end
+
+function labels(family::Family{C}, labelvalues::LabelValues) where C
+    collector = @lock family.lock get!(family.children, labelvalues) do
+        C(nothing, family.metric_name, family.help)
+    end
+    return collector
+end
+labels(family::Family, labelvalues) = labels(family, LabelValues(labelvalues))
+
+function remove(family::Family, labelvalues::LabelValues)
+    @lock family.lock delete!(family.children, labelvalues)
+    return
+end
+remove(family::Family, labelvalues) = remove(family, LabelValues(labelvalues))
+
+function clear(family::Family)
+    @lock family.lock empty!(family.children)
+    return
+end
+
+prometheus_type(::Type{Counter}) = "counter"
+prometheus_type(::Type{Gauge}) = "gauge"
+prometheus_type(::Type{Summary}) = "summary"
+prometheus_type(::Type) = unreachable()
+
+function collect!(metrics::Vector, family::Family{C}) where C
+    type = prometheus_type(C)
+    samples = Sample[]
+    buf = Metric[]
+    @lock family.lock begin
+        for (labels, child) in family.children
+            # collect!(...) the child, throw away the metric, but keep the samples
+            child_metrics = collect!(resize!(buf, 0), child)
+            length(child_metrics) !=1 && error("multiple metrics not supported (yet?)")
+            child_metric = child_metrics[1]
+            @assert(child_metric.type == type)
+            # Unwrap and rewrap samples with the labels
+            child_samples = child_metric.samples
+            if child_samples isa Sample
+                push!(samples, Sample(child_samples.suffix, labels, child_samples.value))
+            else
+                @assert(child_samples isa Vector{Sample})
+                for child_sample in child_samples
+                    @assert(child_sample.labels === nothing)
+                    push!(samples, Sample(child_sample.suffix, labels, child_sample.value))
+                end
+            end
+        end
+    end
+    # Sort samples lexicographically by the labels
+    sort!(samples; by = function(x)
+        labels = x.labels
+        @assert(labels !== nothing)
+        return labels.labelvalues
+    end)
+    push!(metrics, Metric(type, family.metric_name, family.help, family.labelnames, samples))
+    return metrics
+end
+
+
+##############
+# Exposition #
+##############
+
+struct Sample
+    suffix::Union{String, Nothing} # e.g. _count or _sum
+    labels::Union{LabelValues, Nothing}
+    value::Float64
+end
+
+struct Metric
+    type::String
+    metric_name::String
+    help::String
+    labelnames::Union{LabelNames, Nothing}
+    # TODO: Union{Tuple{Sample}, Vector{Sample}} would always make this iterable.
+    samples::Union{Sample, Vector{Sample}}
+end
+
+function expose_metric(io::IO, metric::Metric)
+    println(io, "# HELP ", metric.metric_name, " ", metric.help)
+    println(io, "# TYPE ", metric.metric_name, " ", metric.type)
+    labelnames = metric.labelnames
+    samples = metric.samples
+    if samples isa Sample
+        # Single sample, no labels
+        @assert(labelnames === nothing)
+        @assert(samples.labels === nothing)
+        @assert(samples.suffix === nothing)
+        val = samples.value
+        println(io, metric.metric_name, " ", isinteger(val) ? Int(val) : val)
+    else
+        # Multiple samples, might have labels
+        @assert(samples isa Vector{Sample})
+        for sample in samples
+            # Print metric name
+            print(io, metric.metric_name)
+            # Print potential suffix
+            if sample.suffix !== nothing
+                print(io, sample.suffix)
+            end
+            # Print potential labels
+            labels = sample.labels
+            if labelnames !== nothing && labels !== nothing
+                first = true
+                print(io, "{")
+                for (name, value) in zip(labelnames.labelnames, labels.labelvalues)
+                    first || print(io, ",")
+                    print(io, name, "=\"", value, "\"")
+                    first = false
+                end
+                print(io, "}")
+            end
+            # Print the value
+            println(io, " ", isinteger(sample.value) ? Int(sample.value) : sample.value)
+        end
+    end
+end
+
+"""
+    Prometheus.expose(file::String, reg::CollectorRegistry = DEFAULT_REGISTRY)
+
+Export all metrics in `reg` by writing them to the file `file`.
+"""
+function expose(path::String, reg::CollectorRegistry = DEFAULT_REGISTRY)
+    dir = dirname(path)
+    mkpath(dir)
+    mktemp(dirname(path)) do tmp_path, tmp_io
+        expose_io(tmp_io, reg)
+        close(tmp_io)
+        mv(tmp_path, path; force=true)
+    end
+    return
+end
+
+"""
+    expose(io::IO, reg::CollectorRegistry = DEFAULT_REGISTRY)
+
+Export all metrics in `reg` by writing them to the I/O stream `io`.
+"""
+function expose(io::IO, reg::CollectorRegistry = DEFAULT_REGISTRY)
+    return expose_io(io, reg)
+end
+
+function expose_io(io::IO, reg::CollectorRegistry)
+    # Collect all metrics
+    metrics = Metric[]
+    @lock reg.lock for collector in reg.collectors
+        collect!(metrics, collector)
+    end
+    sort!(metrics; by = metric -> metric.metric_name)
+    # Write to IO
+    buf = IOBuffer(; maxsize=1024^2) # 1 MB
+    for metric in metrics
+        truncate(buf, 0)
+        expose_metric(buf, metric)
+        seekstart(buf)
+        write(io, buf)
+    end
+    return
+end
+
+#######################
+# HTTP.jl integration #
+#######################
+
+const CONTENT_TYPE_LATEST = "text/plain; version=0.0.4; charset=utf-8"
+
+"""
+    expose(http::HTTP.Stream, reg::CollectorRegistry = DEFAULT_REGISTRY)
+
+Export all metrics in `reg` by writing them to the HTTP stream `http`.
+
+The caller is responsible for checking e.g. the HTTP method and URI target. For
+HEAD requests this method do not write a body, however.
+"""
+function expose(http::HTTP.Stream, reg::CollectorRegistry = DEFAULT_REGISTRY)
+    # TODO: Handle Accept request header
+    # TODO: Compression if requested/supported
+    HTTP.setstatus(http, 200)
+    HTTP.setheader(http, "Content-Type" => CONTENT_TYPE_LATEST)
+    HTTP.startwrite(http)
+    # The user is repsonsible for making sure that e.g. target and method is
+    # correct, but at least we skip writing the body for HEAD requests.
+    if http.message.method != "HEAD"
+        expose_io(http, reg)
+    end
+    return
+end
+
+end # module Prometheus
diff --git a/test/runtests.jl b/test/runtests.jl
new file mode 100644
index 0000000..287eb7a
--- /dev/null
+++ b/test/runtests.jl
@@ -0,0 +1,352 @@
+using HTTP: HTTP
+using Prometheus: Prometheus
+using Test: @test, @test_throws, @testset
+
+@testset "Prometheus.CollectorRegistry" begin
+    empty!(Prometheus.DEFAULT_REGISTRY.collectors)
+    # Default registry
+    c = Prometheus.Counter("metric_name_counter", "A counter.")
+    @test c in Prometheus.DEFAULT_REGISTRY.collectors
+    @test_throws ErrorException Prometheus.Counter("metric_name_counter", "A counter.")
+    Prometheus.unregister(Prometheus.DEFAULT_REGISTRY, c)
+    @test !(c in Prometheus.DEFAULT_REGISTRY.collectors)
+    c2 = Prometheus.Counter("metric_name_counter", "A counter.")
+    @test c2 in Prometheus.DEFAULT_REGISTRY.collectors
+    # Provided registry
+    r = Prometheus.CollectorRegistry()
+    c = Prometheus.Counter(r, "metric_name_counter", "A counter.")
+    @test c in r.collectors
+    @test !(c in Prometheus.DEFAULT_REGISTRY.collectors)
+    # No registry on construction, register after
+    c = Prometheus.Counter(nothing, "metric_name_counter", "A counter.")
+    @test !(c in Prometheus.DEFAULT_REGISTRY.collectors)
+    r = Prometheus.CollectorRegistry()
+    Prometheus.register(r, c)
+    @test c in r.collectors
+    @test_throws ErrorException Prometheus.register(r, c)
+end
+
+@testset "Prometheus.Counter" begin
+    # Constructors and implicit registration
+    empty!(Prometheus.DEFAULT_REGISTRY.collectors)
+    c = Prometheus.Counter("metric_name_counter", "A counter.")
+    @test c in Prometheus.DEFAULT_REGISTRY.collectors
+    r = Prometheus.CollectorRegistry()
+    c = Prometheus.Counter(r, "metric_name_counter", "A counter.")
+    @test c in r.collectors
+    @test c.value == 0
+    # Prometheus.inc(...)
+    Prometheus.inc(c)
+    @test c.value == 1
+    Prometheus.inc(c, 0)
+    @test c.value == 1
+    Prometheus.inc(c, 2)
+    @test c.value == 3
+    @test_throws ErrorException Prometheus.inc(c, -1)
+    # Prometheus.collect(...)
+    metrics = Prometheus.collect(c)
+    @test length(metrics) == 1
+    metric = metrics[1]
+    @test metric.metric_name == c.metric_name
+    @test metric.help == c.help
+    @test metric.samples.value == c.value
+    # Prometheus.expose_metric(...)
+    @test sprint(Prometheus.expose_metric, metric) ==
+          sprint(Prometheus.expose_io, r) ==
+          """
+          # HELP metric_name_counter A counter.
+          # TYPE metric_name_counter counter
+          metric_name_counter 3
+          """
+end
+
+@testset "Prometheus.Gauge" begin
+    # Constructors and implicit registration
+    empty!(Prometheus.DEFAULT_REGISTRY.collectors)
+    c = Prometheus.Gauge("metric_name_gauge", "A gauge.")
+    @test c in Prometheus.DEFAULT_REGISTRY.collectors
+    r = Prometheus.CollectorRegistry()
+    c = Prometheus.Gauge(r, "metric_name_gauge", "A gauge.")
+    @test c in r.collectors
+    @test c.value == 0
+    # Prometheus.inc(...)
+    Prometheus.inc(c)
+    @test c.value == 1
+    Prometheus.inc(c, 0)
+    @test c.value == 1
+    Prometheus.inc(c, 2)
+    @test c.value == 3
+    @test_throws ErrorException Prometheus.inc(c, -1)
+    # Prometheus.dec(...)
+    Prometheus.dec(c)
+    @test c.value == 2
+    Prometheus.dec(c, 1)
+    @test c.value == 1
+    @test_throws ErrorException Prometheus.dec(c, -1)
+    # Prometheus.set_to_current_time(...)
+    t0 = time()
+    sleep(0.1)
+    Prometheus.set_to_current_time(c)
+    sleep(0.1)
+    @test t0 < c.value < time()
+    # Prometheus.set(...)
+    Prometheus.set(c, 42)
+    @test c.value == 42
+    # Prometheus.collect(...)
+    metrics = Prometheus.collect(c)
+    @test length(metrics) == 1
+    metric = metrics[1]
+    @test metric.metric_name == c.metric_name
+    @test metric.help == c.help
+    @test metric.samples.value == c.value
+    # Prometheus.expose_metric(...)
+    @test sprint(Prometheus.expose_metric, metric) ==
+          sprint(Prometheus.expose_io, r) ==
+          """
+          # HELP metric_name_gauge A gauge.
+          # TYPE metric_name_gauge gauge
+          metric_name_gauge 42
+          """
+end
+
+@testset "Prometheus.Summary" begin
+    # Constructors and implicit registration
+    empty!(Prometheus.DEFAULT_REGISTRY.collectors)
+    c = Prometheus.Summary("metric_name_summary", "A summary.")
+    @test c in Prometheus.DEFAULT_REGISTRY.collectors
+    r = Prometheus.CollectorRegistry()
+    c = Prometheus.Summary(r, "metric_name_summary", "A summary.")
+    @test c in r.collectors
+    @test c._count == 0
+    @test c._sum == 0
+    # Prometheus.observe(...)
+    Prometheus.observe(c, 1)
+    @test c._count == 1
+    @test c._sum == 1
+    Prometheus.observe(c, 10)
+    @test c._count == 2
+    @test c._sum == 11
+    # Prometheus.collect(...)
+    metrics = Prometheus.collect(c)
+    @test length(metrics) == 1
+    metric = metrics[1]
+    @test metric.metric_name == c.metric_name
+    @test metric.help == c.help
+    @test length(metric.samples) == 2
+    s1, s2 = metric.samples[1], metric.samples[2]
+    @test s1.suffix == "_count"
+    @test s2.suffix == "_sum"
+    @test s1.labels === nothing
+    @test s2.labels === nothing
+    @test s1.value == 2
+    @test s2.value == 11
+    # Prometheus.expose_metric(...)
+    @test sprint(Prometheus.expose_metric, metric) ==
+          sprint(Prometheus.expose_io, r) ==
+          """
+          # HELP metric_name_summary A summary.
+          # TYPE metric_name_summary summary
+          metric_name_summary_count 2
+          metric_name_summary_sum 11
+          """
+end
+
+@testset "Prometheus.LabelNames and Prometheus.LabelValues" begin
+    # Custom hashing
+    v1 = Prometheus.LabelValues(["foo", "bar"])
+    v2 = Prometheus.LabelValues(["foo", "bar"])
+    v3 = Prometheus.LabelValues(["foo", "baz"])
+    @test hash(v1) == hash(v2)
+    @test hash(v1) != hash(v3)
+    @test v1 == v2
+    @test v1 != v3
+    @test isequal(v1, v2)
+    @test !isequal(v1, v3)
+end
+
+@testset "Prometheus.Family{$(Collector)}" for Collector in (Prometheus.Counter, Prometheus.Gauge)
+    # Constructors and implicit registration
+    empty!(Prometheus.DEFAULT_REGISTRY.collectors)
+    c = Prometheus.Family{Collector}(
+        "http_requests", "Number of HTTP requests.",
+        ["endpoint", "status_code"],
+    )
+    @test c in Prometheus.DEFAULT_REGISTRY.collectors
+    r = Prometheus.CollectorRegistry()
+    c = Prometheus.Family{Collector}(
+        r, "http_requests", "Number of HTTP requests.",
+        ["endpoint", "status_code"],
+    )
+    @test c in r.collectors
+    @test length(c.children) == 0
+    # Prometheus.labels(...), Prometheus.remove(...), Prometheus.clear()
+    l1 = ["/foo/", "200"]
+    l2 = ["/bar/", "404"]
+    @test Prometheus.labels(c, l1) === Prometheus.labels(c, l1)
+    @test Prometheus.labels(c, l2) === Prometheus.labels(c, l2)
+    @test length(c.children) == 2
+    @test Prometheus.labels(c, l1).value == 0
+    @test Prometheus.labels(c, l2).value == 0
+    Prometheus.remove(c, l1)
+    @test length(c.children) == 1
+    Prometheus.clear(c)
+    @test length(c.children) == 0
+    # Prometheus.inc(...)
+    Prometheus.inc(Prometheus.labels(c, l1))
+    Prometheus.inc(Prometheus.labels(c, l2))
+    @test Prometheus.labels(c, l1).value == 1
+    @test Prometheus.labels(c, l2).value == 1
+    Prometheus.inc(Prometheus.labels(c, l1), 2)
+    Prometheus.inc(Prometheus.labels(c, l2), 2)
+    @test Prometheus.labels(c, l1).value == 3
+    @test Prometheus.labels(c, l2).value == 3
+    # Prometheus.collect(...)
+    metrics = Prometheus.collect(c)
+    @test length(metrics) == 1
+    metric = metrics[1]
+    @test metric.metric_name == c.metric_name
+    @test metric.help == c.help
+    @test length(metric.samples) == 2
+    s1, s2 = metric.samples[1], metric.samples[2]
+    @test s1.labels.labelvalues == ["/bar/", "404"]
+    @test s2.labels.labelvalues == ["/foo/", "200"]
+    @test s1.value == 3
+    @test s2.value == 3
+    # Prometheus.expose_metric(...)
+    type = Collector === Prometheus.Counter ? "counter" : "gauge"
+    @test sprint(Prometheus.expose_metric, metric) ==
+          sprint(Prometheus.expose_io, r) ==
+          """
+          # HELP http_requests Number of HTTP requests.
+          # TYPE http_requests $(type)
+          http_requests{endpoint="/bar/",status_code="404"} 3
+          http_requests{endpoint="/foo/",status_code="200"} 3
+          """
+end
+
+@testset "Prometheus.Family{Summary}" begin
+    r = Prometheus.CollectorRegistry()
+    c = Prometheus.Family{Prometheus.Summary}(
+        r, "http_request_time", "Time to process requests.",
+        ["endpoint", "status_code"],
+    )
+    @test c in r.collectors
+    @test length(c.children) == 0
+    # Prometheus.inc(...)
+    l1 = ["/foo/", "200"]
+    l2 = ["/bar/", "404"]
+    @test Prometheus.labels(c, l1) === Prometheus.labels(c, l1)
+    @test Prometheus.labels(c, l2) === Prometheus.labels(c, l2)
+    @test length(c.children) == 2
+    @test Prometheus.labels(c, l1)._count == 0
+    @test Prometheus.labels(c, l1)._sum == 0
+    @test Prometheus.labels(c, l2)._count == 0
+    @test Prometheus.labels(c, l2)._sum == 0
+    Prometheus.observe(Prometheus.labels(c, l1), 1.2)
+    Prometheus.observe(Prometheus.labels(c, l2), 2.1)
+    @test Prometheus.labels(c, l1)._count == 1
+    @test Prometheus.labels(c, l1)._sum == 1.2
+    @test Prometheus.labels(c, l2)._count == 1
+    @test Prometheus.labels(c, l2)._sum == 2.1
+    Prometheus.observe(Prometheus.labels(c, l1), 3.4)
+    Prometheus.observe(Prometheus.labels(c, l2), 4.3)
+    @test Prometheus.labels(c, l1)._count == 2
+    @test Prometheus.labels(c, l1)._sum == 4.6
+    @test Prometheus.labels(c, l2)._count == 2
+    @test Prometheus.labels(c, l2)._sum == 6.4
+    # Prometheus.collect(...)
+    metrics = Prometheus.collect(c)
+    @test length(metrics) == 1
+    metric = metrics[1]
+    @test metric.metric_name == c.metric_name
+    @test metric.help == c.help
+    @test length(metric.samples) == 4
+    s1, s2, s3, s4 = metric.samples
+    @test s1.labels.labelvalues == s2.labels.labelvalues == ["/bar/", "404"]
+    @test s3.labels.labelvalues == s4.labels.labelvalues == ["/foo/", "200"]
+    @test s1.value == 2   # _count
+    @test s2.value == 6.4 # _sum
+    @test s3.value == 2   # _count
+    @test s4.value == 4.6 # _sum
+    # Prometheus.expose_metric(...)
+    @test sprint(Prometheus.expose_metric, metric) ==
+          sprint(Prometheus.expose_io, r) ==
+          """
+          # HELP http_request_time Time to process requests.
+          # TYPE http_request_time summary
+          http_request_time_count{endpoint="/bar/",status_code="404"} 2
+          http_request_time_sum{endpoint="/bar/",status_code="404"} 6.4
+          http_request_time_count{endpoint="/foo/",status_code="200"} 2
+          http_request_time_sum{endpoint="/foo/",status_code="200"} 4.6
+          """
+end
+
+@testset "Prometheus.expose(::Union{String, IO})" begin
+    r = Prometheus.DEFAULT_REGISTRY
+    empty!(r.collectors)
+    Prometheus.inc(Prometheus.Counter(r, "prom_counter", "Counting things"))
+    Prometheus.set(Prometheus.Gauge(r, "prom_gauge", "Gauging things"), 1.2)
+    mktempdir() do dir
+        default = joinpath(dir, "default.prom")
+        Prometheus.expose(default)
+        reg = joinpath(dir, "reg.prom")
+        Prometheus.expose(reg, r)
+        default_io = IOBuffer()
+        Prometheus.expose(default_io)
+        reg_io = IOBuffer()
+        Prometheus.expose(reg_io, r)
+        @test read(default, String) ==
+              read(reg, String) ==
+              String(take!(default_io)) ==
+              String(take!(reg_io)) ==
+              """
+              # HELP prom_counter Counting things
+              # TYPE prom_counter counter
+              prom_counter 1
+              # HELP prom_gauge Gauging things
+              # TYPE prom_gauge gauge
+              prom_gauge 1.2
+              """
+    end
+end
+
+@testset "Prometheus.expose(::HTTP.Stream)" begin
+    empty!(Prometheus.DEFAULT_REGISTRY.collectors)
+    Prometheus.inc(Prometheus.Counter("prom_counter", "Counting things"))
+    Prometheus.set(Prometheus.Gauge("prom_gauge", "Gauging things"), 1.2)
+    iob = IOBuffer()
+    Prometheus.expose(iob)
+    reference_output = String(take!(iob))
+    # Spin up the server
+    server = HTTP.listen!(8123) do http
+        if http.message.target == "/metrics/default"
+            return Prometheus.expose(http)
+        elseif http.message.target == "/metrics/reg"
+            return Prometheus.expose(http, Prometheus.DEFAULT_REGISTRY)
+        else
+            HTTP.setstatus(http, 404)
+            HTTP.startwrite(http)
+        end
+    end
+    # Normal requests
+    r_default = HTTP.request("GET", "http://localhost:8123/metrics/default")
+    r_ref = HTTP.request("GET", "http://localhost:8123/metrics/reg")
+    @test String(r_default.body) == String(r_ref.body) == reference_output
+    # HEAD
+    @test isempty(HTTP.request("HEAD", "http://localhost:8123/metrics/default").body)
+    # POST (no filtering in the server above)
+    r_post = HTTP.request("POST", "http://localhost:8123/metrics/default")
+    @test String(r_post.body) == reference_output
+    # Bad URI
+    r_bad = HTTP.request("GET", "http://localhost:8123"; status_exception=false)
+    @test r_bad.status == 404
+    # Clean up
+    close(server)
+    wait(server)
+end
+
+@testset "Utilities" begin
+    @test_throws Prometheus.PrometheusUnreachable Prometheus.unreachable()
+    @test_throws Prometheus.PrometheusAssert      Prometheus.@assert false
+    @test_throws Prometheus.PrometheusUnreachable Prometheus.prometheus_type(Int)
+end