Overloaded (#1364)

* plugin/health: add 'overloaded metrics'

Query our on health endpoint and record (and export as a metric) the
time it takes. The Get has a 5s timeout, that, when reached, will set
the metric duration to 5s. The actually call "I'm I overloaded" is left
to an external entity.

* README

* golint and govet

* and the tests

plugin/health/overloaded.go

@@ -0,0 +1,52 @@
+package health
+
+import (
+	"net/http"
+	"sync"
+	"time"
+
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// overloaded queries the health end point and updates a metrics showing how long it took.
+func (h *health) overloaded() {
+	timeout := time.Duration(5 * time.Second)
+	client := http.Client{
+		Timeout: timeout,
+	}
+	url := "http://" + h.Addr
+	tick := time.NewTicker(1 * time.Second)
+
+	for {
+		select {
+		case <-tick.C:
+			start := time.Now()
+			resp, err := client.Get(url)
+			if err != nil {
+				HealthDuration.Observe(timeout.Seconds())
+				continue
+			}
+			resp.Body.Close()
+			HealthDuration.Observe(time.Since(start).Seconds())
+
+		case <-h.stop:
+			tick.Stop()
+			return
+		}
+	}
+}
+
+var (
+	// HealthDuration is the metric used for exporting how fast we can retrieve the /health endpoint.
+	HealthDuration = prometheus.NewHistogram(prometheus.HistogramOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: "health",
+		Name:      "request_duration_seconds",
+		Buckets:   plugin.TimeBuckets,
+		Help:      "Histogram of the time (in seconds) each request took.",
+	})
+)
+
+var onceMetric sync.Once