prometheus: implement Collector interface for Registry

This change allows Registries to be used as Collectors.

This enables new instances of Registry to be passed to ephemeral
subroutines for collecting metrics from subroutines which are still
running:

```go
package main

import (
  "fmt"

  "github.com/prometheus/client_golang/prometheus"
)

func main() {
  globalReg := prometheus.NewRegistry()

  for i := 0; i < 100; i++ {
    workerReg := prometheus.WrapRegistererWith(prometheus.Labels{
      // Add an ID label so registered metrics from workers don't
      // collide.
      "worker_id": fmt.Sprintf("%d", i),
    }, prometheus.NewRegistry()

    globalReg.MustRegister(workerReg)

    go func(i int) {
      runWorker(workerReg)

      // Unregister any metrics the worker may have created.
      globalReg.Unregister(workerReg)
    }(i)
  }
}

// runWorker runs a worker, registering worker-specific metrics.
func runWorker(reg *prometheus.Registry) {
  // ... register metrics ...
  // ... do work ...
}
```

This change makes it easier to avoid leaking metrics from subroutines
which do not consistently properly unregister metrics.

prometheus/registry.go

@@ -252,9 +252,9 @@ func (errs MultiError) MaybeUnwrap() error {
 }
 
 // Registry registers Prometheus collectors, collects their metrics, and gathers
-// them into MetricFamilies for exposition. It implements both Registerer and
-// Gatherer. The zero value is not usable. Create instances with NewRegistry or
-// NewPedanticRegistry.
+// them into MetricFamilies for exposition. It implements both Registerer,
+// Gatherer, and Collector. The zero value is not usable. Create instances with
+// NewRegistry or NewPedanticRegistry.
 type Registry struct {
 	mtx                   sync.RWMutex
 	collectorsByID        map[uint64]Collector // ID is a hash of the descIDs.
@@ -556,6 +556,31 @@ func (r *Registry) Gather() ([]*dto.MetricFamily, error) {
 	return internal.NormalizeMetricFamilies(metricFamiliesByName), errs.MaybeUnwrap()
 }
 
+// Describe implements Collector.
+func (r *Registry) Describe(ch chan<- *Desc) {
+	r.mtx.RLock()
+	defer r.mtx.RUnlock()
+
+	// Only report the checked Collectors; unchecked collectors don't report any
+	// Desc.
+	for _, c := range r.collectorsByID {
+		c.Describe(ch)
+	}
+}
+
+// Collect implements Collector.
+func (r *Registry) Collect(ch chan<- Metric) {
+	r.mtx.RLock()
+	defer r.mtx.RUnlock()
+
+	for _, c := range r.collectorsByID {
+		c.Collect(ch)
+	}
+	for _, c := range r.uncheckedCollectors {
+		c.Collect(ch)
+	}
+}
+
 // WriteToTextfile calls Gather on the provided Gatherer, encodes the result in the
 // Prometheus text format, and writes it to a temporary file. Upon success, the
 // temporary file is renamed to the provided filename.