plugin/reload: Add metrics (#2922)

* Issue 2920
- Add failed reload counter metrics

* typo fix

* Requested Changes

plugin/reload/README.md

@@ -92,6 +92,12 @@ In general be careful with assigning new port and expecting reload to work fully
 Also any `import` statement is not discovered by this plugin. This means if any of these imported files
 changes the *reload* plugin is ignorant of that fact.
 
+## Metrics
+
+ If monitoring is enabled (via the *prometheus* directive) then the following metric is exported:
+
+* `coredns_reload_failed_count_total{}` - counts the number of failed reload attempts.
+
 ## Also See
 
 See coredns-import(7) and corefile(5).

plugin/reload/metrics.go

+package reload
+
+import (
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// Metrics for the reload plugin
+var (
+	FailedCount = prometheus.NewCounter(prometheus.CounterOpts{
+		Namespace: plugin.Namespace,
+		Subsystem: "reload",
+		Name:      "failed_count_total",
+		Help:      "Counter of the number of failed reload attempts.",
+	})
+)

plugin/reload/reload.go

@@ -83,6 +83,7 @@ func hook(event caddy.EventName, info interface{}) error {
 					_, err := instance.Restart(corefile)
 					if err != nil {
 						log.Errorf("Corefile changed but reload failed: %s", err)
+						FailedCount.Add(1)
 						continue
 					}
 					// we are done, if the plugin was not set used, then it is not.