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>

vendor/github.com/docker/go-metrics/helpers.go

@@ -0,0 +1,10 @@
+package metrics
+
+func sumFloat64(vs ...float64) float64 {
+	var sum float64
+	for _, v := range vs {
+		sum += v
+	}
+
+	return sum
+}