Add Prometheus Metrics

at the first iteration, only the following metrics are collected:

  - HTTP metrics of each API endpoint
  - cache counter for request/hit/miss
  - histogram of storage actions, including:
    GetContent, PutContent, Stat, List, Move, and Delete

Signed-off-by: tifayuki <tifayuki@gmail.com>

registry/storage/cache/cachedblobdescriptorstore.go

@@ -4,6 +4,7 @@ import (
 	"context"
 
 	"github.com/docker/distribution"
+	prometheus "github.com/docker/distribution/metrics"
 	"github.com/opencontainers/go-digest"
 )
 
@@ -38,6 +39,11 @@ type cachedBlobStatter struct {
 	tracker MetricsTracker
 }
 
+var (
+	// cacheCount is the number of total cache request received/hits/misses
+	cacheCount = prometheus.StorageNamespace.NewLabeledCounter("cache", "The number of cache request received", "type")
+)
+
 // NewCachedBlobStatter creates a new statter which prefers a cache and
 // falls back to a backend.
 func NewCachedBlobStatter(cache distribution.BlobDescriptorService, backend distribution.BlobDescriptorService) distribution.BlobDescriptorService {
@@ -58,6 +64,7 @@ func NewCachedBlobStatterWithMetrics(cache distribution.BlobDescriptorService, b
 }
 
 func (cbds *cachedBlobStatter) Stat(ctx context.Context, dgst digest.Digest) (distribution.Descriptor, error) {
+	cacheCount.WithValues("Request").Inc(1)
 	desc, err := cbds.cache.Stat(ctx, dgst)
 	if err != nil {
 		if err != distribution.ErrBlobUnknown {
@@ -66,12 +73,13 @@ func (cbds *cachedBlobStatter) Stat(ctx context.Context, dgst digest.Digest) (di
 
 		goto fallback
 	}
-
+	cacheCount.WithValues("Hit").Inc(1)
 	if cbds.tracker != nil {
 		cbds.tracker.Hit()
 	}
 	return desc, nil
 fallback:
+	cacheCount.WithValues("Miss").Inc(1)
 	if cbds.tracker != nil {
 		cbds.tracker.Miss()
 	}