plugin/metrics: Add a metric to monitor which plugin(s) is(are) enabled (#2700)

* Add a GaugeVec for enabled plugins monitoring.

Signed-off-by: Jiacheng Xu <xjcmaxwellcjx@gmail.com>

* Add server label and zone label for enable_plugin matric.

* Add a test for PluginEnabled metric

* Add description for enabledPlugin metric.

* Change the description for the enabledPlugin metric.

* Reset the enabledPlugin metric when restart the server.

* Add the bug session for enabledPlugin metric.

* Remove the resolveTCPAddr

plugin/metrics/vars/vars.go

@@ -65,6 +65,12 @@ var (
 		Name:      "panic_count_total",
 		Help:      "A metrics that counts the number of panics.",
 	})
+
+	PluginEnabled = prometheus.NewGaugeVec(prometheus.GaugeOpts{
+		Namespace: plugin.Namespace,
+		Name:      "plugin_enabled",
+		Help:      "A metric that indicates whether a plugin is enabled on per server and zone basis.",
+	}, []string{"server", "zone", "name"})
 )
 
 const (