docs(plugins) add link to Grafana dashboard to Prometheus plugin

From #816

app/plugins/prometheus.md

@@ -8,9 +8,9 @@ breadcrumbs:
 nav:
   - label: Usage
     items:
-      - label: Reading metrics
-      - lable: Available metrics
-      - lable: Accessing the metrics
+      - label: Grafana dashboard
+      - label: Available metrics
+      - label: Accessing the metrics
 description: |
     Expose metrics related to Kong and proxied upstream services in [Prometheus](https://prometheus.io/docs/introduction/overview/) exposition format, which can be scrapped by a Prometheus Server.
 
@@ -22,8 +22,6 @@ params:
 
 ---
 
-### Reading metrics
-
 Metrics are available on the Admin API at the `http://localhost:8001/metrics`
 endpoint. Note that the URL to the Admin API will be specific to your
 installation; see _Accessing the metrics_ below.
@@ -36,7 +34,12 @@ specifying `admin_listen = off`) will need to use a [custom Nginx
 configuration template](/latest/configuration/#custom-nginx-configuration)
 to expose the metrics data.
 
-#### Available metrics
+### Grafana dashboard
+
+Metrics exported by the plugin can be graphed in Grafana using a drop in
+dashboard: [https://grafana.com/dashboards/7424](https://grafana.com/dashboards/7424).
+
+### Available metrics
 
 - **Status codes**: HTTP status codes returned by upstream services.
   These are available per service and across all services.