Counter+Gauge+Summary: add method references.

src/Prometheus.jl

@@ -138,6 +138,9 @@ Construct a Counter collector.
  - `registry :: Prometheus.CollectorRegistry`: the registry in which to register the
    collector. If not specified the default registry is used. Pass `registry = nothing` to
    skip registration.
+
+**Methods**
+ - [`Prometheus.inc`](@ref): increment the counter.
 """
 Counter(::String, ::String; kwargs...)
 
@@ -214,6 +217,14 @@ Construct a Gauge collector.
  - `registry :: Prometheus.CollectorRegistry`: the registry in which to register the
    collector. If not specified the default registry is used. Pass `registry = nothing` to
    skip registration.
+
+**Methods**
+ - [`Prometheus.inc`](@ref inc(::Gauge, ::Any)): increment the value
+   of the gauge.
+ - [`Prometheus.dec`](@ref): decrement the value of the gauge.
+ - [`Prometheus.set`](@ref): set the value of the gauge.
+ - [`Prometheus.set_to_current_time`](@ref): set the value of the gauge to the
+   current unixtime.
 """
 Gauge(::String, ::String; kwargs...)
 
@@ -318,6 +329,9 @@ Construct a Summary collector.
  - `registry :: Prometheus.CollectorRegistry`: the registry in which to register the
    collector. If not specified the default registry is used. Pass `registry = nothing` to
    skip registration.
+
+**Methods**
+ - [`Prometheus.observe`](@ref): add an observation to the summary.
 """
 Summary(::String, ::String; kwargs...)