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/unit.go

@@ -0,0 +1,12 @@
+package metrics
+
+// Unit represents the type or precision of a metric that is appended to
+// the metrics fully qualified name
+type Unit string
+
+const (
+	Nanoseconds Unit = "nanoseconds"
+	Seconds     Unit = "seconds"
+	Bytes       Unit = "bytes"
+	Total       Unit = "total"
+)