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/handler.go

@@ -0,0 +1,74 @@
+package metrics
+
+import (
+	"net/http"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+// HTTPHandlerOpts describes a set of configurable options of http metrics
+type HTTPHandlerOpts struct {
+	DurationBuckets     []float64
+	RequestSizeBuckets  []float64
+	ResponseSizeBuckets []float64
+}
+
+const (
+	InstrumentHandlerResponseSize = iota
+	InstrumentHandlerRequestSize
+	InstrumentHandlerDuration
+	InstrumentHandlerCounter
+	InstrumentHandlerInFlight
+)
+
+type HTTPMetric struct {
+	prometheus.Collector
+	handlerType int
+}
+
+var (
+	defaultDurationBuckets     = []float64{.005, .01, .025, .05, .1, .25, .5, 1, 2.5, 5, 10, 25, 60}
+	defaultRequestSizeBuckets  = prometheus.ExponentialBuckets(1024, 2, 22) //1K to 4G
+	defaultResponseSizeBuckets = defaultRequestSizeBuckets
+)
+
+// Handler returns the global http.Handler that provides the prometheus
+// metrics format on GET requests. This handler is no longer instrumented.
+func Handler() http.Handler {
+	return promhttp.Handler()
+}
+
+func InstrumentHandler(metrics []*HTTPMetric, handler http.Handler) http.HandlerFunc {
+	return InstrumentHandlerFunc(metrics, handler.ServeHTTP)
+}
+
+func InstrumentHandlerFunc(metrics []*HTTPMetric, handlerFunc http.HandlerFunc) http.HandlerFunc {
+	var handler http.Handler
+	handler = http.HandlerFunc(handlerFunc)
+	for _, metric := range metrics {
+		switch metric.handlerType {
+		case InstrumentHandlerResponseSize:
+			if collector, ok := metric.Collector.(prometheus.ObserverVec); ok {
+				handler = promhttp.InstrumentHandlerResponseSize(collector, handler)
+			}
+		case InstrumentHandlerRequestSize:
+			if collector, ok := metric.Collector.(prometheus.ObserverVec); ok {
+				handler = promhttp.InstrumentHandlerRequestSize(collector, handler)
+			}
+		case InstrumentHandlerDuration:
+			if collector, ok := metric.Collector.(prometheus.ObserverVec); ok {
+				handler = promhttp.InstrumentHandlerDuration(collector, handler)
+			}
+		case InstrumentHandlerCounter:
+			if collector, ok := metric.Collector.(*prometheus.CounterVec); ok {
+				handler = promhttp.InstrumentHandlerCounter(collector, handler)
+			}
+		case InstrumentHandlerInFlight:
+			if collector, ok := metric.Collector.(prometheus.Gauge); ok {
+				handler = promhttp.InstrumentHandlerInFlight(collector, handler)
+			}
+		}
+	}
+	return handler.ServeHTTP
+}