Add RED metrics

- this PR adds metrics via Prometheus for instrumentation and
to move towards using HPAv2 / custom metrics in Kubernetes.

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

watchdog/metrics/http.go

@@ -0,0 +1,27 @@
+package metrics
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+type Http struct {
+	RequestsTotal            *prometheus.CounterVec
+	RequestDurationHistogram *prometheus.HistogramVec
+}
+
+func NewHttp() Http {
+	return Http{
+		RequestsTotal: promauto.NewCounterVec(prometheus.CounterOpts{
+			Subsystem: "http",
+			Name:      "requests_total",
+			Help:      "total HTTP requests processed",
+		}, []string{"code", "method"}),
+		RequestDurationHistogram: promauto.NewHistogramVec(prometheus.HistogramOpts{
+			Subsystem: "http",
+			Name:      "request_duration_seconds",
+			Help:      "Seconds spent serving HTTP requests.",
+			Buckets:   prometheus.DefBuckets,
+		}, []string{"code", "method"}),
+	}
+}