Documentation/op-guide: add Grafana dashboard link

Signed-off-by: Gyu-Ho Lee <gyuhox@gmail.com>

Documentation/op-guide/monitoring.md

@@ -117,6 +117,8 @@ Access: proxy
 
 Then import the default [etcd dashboard template][template] and customize. For instance, if Prometheus data source name is `my-etcd`, the `datasource` field values in JSON also need to be `my-etcd`.
 
+See the [demo][demo].
+
 Sample dashboard:
 
 ![](./etcd-sample-grafana.png)
@@ -125,3 +127,4 @@ Sample dashboard:
 [prometheus]: https://prometheus.io/
 [grafana]: http://grafana.org/
 [template]: ./grafana.json
+[demo]: http://dash.etcd.io/dashboard/db/test-etcd-kubernetes