code: func NewCachedBlobStatterWithMetrics(cache distribution.BlobDescriptorService, backend distribution.BlobDescriptorService, tracker MetricsTracker) distribution.BlobStatter {
  	return &cachedBlobStatter{
  		cache:   cache,
  		backend: backend,
  		tracker: tracker,
  	}
  }
docs: // NewCachedBlobStatterWithMetrics creates a new statter which prefers a cache and
  // falls back to a backend. Hits and misses will send to the tracker.