plugin/autopath: Add metrics and remove log line (#1143)

* plugin/autopath: Add namespace selector and metrics

Add a namespace, so autopathing only is performed in this namespace.
This will make caching work for the cluster again.

Also export metrics that we've done a successful autopath

* dont shadow

* Fix

* Back the namespacing changes

plugin/autopath/metrics.go

@@ -0,0 +1,29 @@
+package autopath
+
+import (
+	"sync"
+
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// Metrics for autopath.
+var (
+	AutoPathCount = prometheus.NewCounterVec(prometheus.CounterOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: "autopath",
+		Name:      "success_count_total",
+		Help:      "Counter of requests that did autopath.",
+	}, []string{})
+)
+
+// OnStartupMetrics sets up the metrics on startup.
+func OnStartupMetrics() error {
+	metricsOnce.Do(func() {
+		prometheus.MustRegister(AutoPathCount)
+	})
+	return nil
+}
+
+var metricsOnce sync.Once