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/gauge.go

@@ -0,0 +1,72 @@
+package metrics
+
+import "github.com/prometheus/client_golang/prometheus"
+
+// Gauge is a metric that allows incrementing and decrementing a value
+type Gauge interface {
+	Inc(...float64)
+	Dec(...float64)
+
+	// Add adds the provided value to the gauge's current value
+	Add(float64)
+
+	// Set replaces the gauge's current value with the provided value
+	Set(float64)
+}
+
+// LabeledGauge describes a gauge the must have values populated before use.
+type LabeledGauge interface {
+	WithValues(labels ...string) Gauge
+}
+
+type labeledGauge struct {
+	pg *prometheus.GaugeVec
+}
+
+func (lg *labeledGauge) WithValues(labels ...string) Gauge {
+	return &gauge{pg: lg.pg.WithLabelValues(labels...)}
+}
+
+func (lg *labeledGauge) Describe(c chan<- *prometheus.Desc) {
+	lg.pg.Describe(c)
+}
+
+func (lg *labeledGauge) Collect(c chan<- prometheus.Metric) {
+	lg.pg.Collect(c)
+}
+
+type gauge struct {
+	pg prometheus.Gauge
+}
+
+func (g *gauge) Inc(vs ...float64) {
+	if len(vs) == 0 {
+		g.pg.Inc()
+	}
+
+	g.Add(sumFloat64(vs...))
+}
+
+func (g *gauge) Dec(vs ...float64) {
+	if len(vs) == 0 {
+		g.pg.Dec()
+	}
+
+	g.Add(-sumFloat64(vs...))
+}
+
+func (g *gauge) Add(v float64) {
+	g.pg.Add(v)
+}
+
+func (g *gauge) Set(v float64) {
+	g.pg.Set(v)
+}
+
+func (g *gauge) Describe(c chan<- *prometheus.Desc) {
+	g.pg.Describe(c)
+}
+
+func (g *gauge) Collect(c chan<- prometheus.Metric) {
+	g.pg.Collect(c)
+}