middleware/proxy: add request duration monitoring (#362)

Add a separate request duration metrics specially for proxying requests
upstream.

Fixes #259

middleware/proxy/metrics.go

@@ -0,0 +1,32 @@
+package proxy
+
+import (
+	"sync"
+
+	"github.com/miekg/coredns/middleware"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// Metrics the proxy middleware exports.
+var (
+	RequestDuration = prometheus.NewHistogramVec(prometheus.HistogramOpts{
+		Namespace: middleware.Namespace,
+		Subsystem: subsystem,
+		Name:      "request_duration_milliseconds",
+		Buckets:   append(prometheus.DefBuckets, []float64{50, 100, 200, 500, 1000, 2000, 3000, 4000, 5000, 10000}...),
+		Help:      "Histogram of the time (in milliseconds) each request took.",
+	}, []string{"zone"})
+)
+
+// OnStartup sets up the metrics on startup.
+func OnStartup() error {
+	metricsOnce.Do(func() {
+		prometheus.MustRegister(RequestDuration)
+	})
+	return nil
+}
+
+var metricsOnce sync.Once
+
+const subsystem = "proxy"