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/metrics_test.go

@@ -0,0 +1,21 @@
+package metrics
+
+import (
+	"testing"
+	"time"
+)
+
+func Test_RegisterServer(t *testing.T) {
+
+	metricsPort := 31111
+
+	metricsServer := MetricsServer{}
+	metricsServer.Register(metricsPort)
+
+	cancel := make(chan bool)
+	go metricsServer.Serve(cancel)
+
+	time.AfterFunc(time.Millisecond*500, func() {
+		cancel <- true
+	})
+}