Add metrics

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/traffic/xds/metrics.go

@@ -0,0 +1,17 @@
+package xds
+
+import (
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+var (
+	// ClusterGauge is the number of clusters we are currently tracking.
+	ClusterGauge = prometheus.NewGaugeVec(prometheus.GaugeOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: "traffic",
+		Name:      "clusters_tracked",
+		Help:      "Gauge of tracked clusters.",
+	}, []string{""})
+)