ProcessCollector: add docstring and documentation.

docs/src/index.md

@@ -11,7 +11,7 @@ Two of the basic concepts of a Prometheus client are [Registries](@ref) and
 [Collectors](@ref). Registries are collections of collectors, and the collectors are the
 units responsible to record and capture metrics. Client libraries implement a default
 registry which all collectors implicity register with, so for basic usage there is no need
-to interact with a registry (see [Default collector registry](@ref)).
+to interact with a registry (see [Default registry](@ref)).
 
 The third important concept is [Exposition](@ref) of the collected metrics. Typically
 metrics are exposed over a HTTP server, as in the [Quickstart](@ref)-example just below. See
@@ -138,6 +138,15 @@ metrics about I/O operations. Metrics from this collector have the `process_` pr
 their name. This collector is only available on Linux since it requires the `/proc` file
 system.
 
+A `ProcessCollector` for the current process is registered automatically with the
+default registry, see [Default registry](@ref) for more details.
+
+#### ProcessCollector API reference
+
+```@docs
+Prometheus.ProcessCollector(::Function; kwargs...)
+```
+
 ### Custom collectors
 
 RandomCollector
@@ -168,7 +177,7 @@ Supported methods:
 ## Registries
 
 
-### Default collector registry
+### Default registry
 
 
 ## Exposition

src/process_collector.jl

@@ -53,6 +53,34 @@ mutable struct ProcessCollector <: Collector
     end
 end
 
+"""
+    Prometheus.ProcessCollector(pid; registry=DEFAULT_REGISTRY)
+
+Create a process collector for the process id given by the `pid` function. The collector
+exposes metrics about the process' CPU time, start time, memory usage, file usage, and I/O
+operations.
+
+**Arguments**
+ - `pid :: Function`: a function returning a process id as a string or integer for which to
+   collect metrics. By default the `"self"` pid is used, i.e. the current process.
+**Keyword arguments**
+ - `registry :: Prometheus.CollectorRegistry`: the registry in which to register the
+   collector. The default registry is used by default. Pass `registry = nothing` to skip
+   registration.
+
+!!! note
+    A `ProcessCollector` for the current process is registered automatically with the
+    default registry. If necessary it can be removed by calling
+    ```julia
+    Prometheus.unregister(Prometheus.DEFAULT_REGISTRY, Prometheus.PROCESS_COLLECTOR)
+    ```
+
+!!! note
+    The process collector is currently only available on Linux since it requires the `/proc`
+    file system. On Windows and macOS this collector will not expose any metrics.
+"""
+ProcessCollector(::Function; kwargs...)
+
 function metric_names(::ProcessCollector)
     return (
         "process_cpu_seconds_total", "process_start_time_seconds",