GCCollector: add docstring and documentation.

docs/src/index.md

@@ -131,6 +131,15 @@ A collector that exports metrics about allocations and garbage collection (for e
 number of allocations, number of bytes allocated, time spent in garbage collection, etc).
 These metrics have the `gc_` prefix in their name.
 
+A `GCCollector` is registered automatically with the default registry, see
+[Default registry](@ref) for more details.
+
+#### GCCollector API reference
+
+```@docs
+Prometheus.GCCollector(; kwargs...)
+```
+
 ### ProcessCollector
 
 A collector that exports metrics about a running process, for example CPU seconds and

src/gc_collector.jl

@@ -12,6 +12,25 @@ mutable struct GCCollector <: Collector
     end
 end
 
+"""
+    Prometheus.GCCollector(; registry=DEFAULT_REGISTRY)
+
+Create a collector that exports metrics about allocations and garbage collection.
+
+**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 `GCCollector` is registered automatically with the default registry.
+    If necessary it can be removed by calling
+    ```julia
+    Prometheus.unregister(Prometheus.DEFAULT_REGISTRY, Prometheus.GC_COLLECTOR)
+    ```
+"""
+GCCollector(; kwargs...)
+
 function metric_names(::GCCollector)
     return (
         "gc_alloc_total", "gc_free_total", "gc_alloc_bytes_total",