Cross-link documentation to the prometheus recording rules. (#8667)

changelog.d/8667.doc

+Interlink prometheus/grafana documentation.

contrib/grafana/README.md

@@ -3,4 +3,4 @@
 0. Set up Prometheus and Grafana. Out of scope for this readme. Useful documentation about using Grafana with Prometheus: http://docs.grafana.org/features/datasources/prometheus/
 1. Have your Prometheus scrape your Synapse. https://github.com/matrix-org/synapse/blob/master/docs/metrics-howto.md
 2. Import dashboard into Grafana. Download `synapse.json`. Import it to Grafana and select the correct Prometheus datasource. http://docs.grafana.org/reference/export_import/
-3. Set up additional recording rules
+3. Set up required recording rules. https://github.com/matrix-org/synapse/tree/master/contrib/prometheus

docs/metrics-howto.md

@@ -60,6 +60,8 @@
 
 1.  Restart Prometheus.
 
+1.  Consider using the [grafana dashboard](https://github.com/matrix-org/synapse/tree/master/contrib/grafana/) and required [recording rules](https://github.com/matrix-org/synapse/tree/master/contrib/prometheus/) 
+
 ## Monitoring workers
 
 To monitor a Synapse installation using