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/timer.go

@@ -0,0 +1,85 @@
+package metrics
+
+import (
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// StartTimer begins a timer observation at the callsite. When the target
+// operation is completed, the caller should call the return done func().
+func StartTimer(timer Timer) (done func()) {
+	start := time.Now()
+	return func() {
+		timer.Update(time.Since(start))
+	}
+}
+
+// Timer is a metric that allows collecting the duration of an action in seconds
+type Timer interface {
+	// Update records an observation, duration, and converts to the target
+	// units.
+	Update(duration time.Duration)
+
+	// UpdateSince will add the duration from the provided starting time to the
+	// timer's summary with the precisions that was used in creation of the timer
+	UpdateSince(time.Time)
+}
+
+// LabeledTimer is a timer that must have label values populated before use.
+type LabeledTimer interface {
+	WithValues(labels ...string) *labeledTimerObserver
+}
+
+type labeledTimer struct {
+	m *prometheus.HistogramVec
+}
+
+type labeledTimerObserver struct {
+	m prometheus.Observer
+}
+
+func (lbo *labeledTimerObserver) Update(duration time.Duration) {
+	lbo.m.Observe(duration.Seconds())
+}
+
+func (lbo *labeledTimerObserver) UpdateSince(since time.Time) {
+	lbo.m.Observe(time.Since(since).Seconds())
+}
+
+func (lt *labeledTimer) WithValues(labels ...string) *labeledTimerObserver {
+	return &labeledTimerObserver{m: lt.m.WithLabelValues(labels...)}
+}
+
+func (lt *labeledTimer) Describe(c chan<- *prometheus.Desc) {
+	lt.m.Describe(c)
+}
+
+func (lt *labeledTimer) Collect(c chan<- prometheus.Metric) {
+	lt.m.Collect(c)
+}
+
+type timer struct {
+	m prometheus.Observer
+}
+
+func (t *timer) Update(duration time.Duration) {
+	t.m.Observe(duration.Seconds())
+}
+
+func (t *timer) UpdateSince(since time.Time) {
+	t.m.Observe(time.Since(since).Seconds())
+}
+
+func (t *timer) Describe(c chan<- *prometheus.Desc) {
+	c <- t.m.(prometheus.Metric).Desc()
+}
+
+func (t *timer) Collect(c chan<- prometheus.Metric) {
+	// Are there any observers that don't implement Collector? It is really
+	// unclear what the point of the upstream change was, but we'll let this
+	// panic if we get an observer that doesn't implement collector. In this
+	// case, we should almost always see metricVec objects, so this should
+	// never panic.
+	t.m.(prometheus.Collector).Collect(c)
+}