r/synapse: Enable Prometheus metrics

Synapse supports exporting metrics in Prometheus format.  It can do this
either as part of the main server, or in a separate listener.  I chose
to use a separate listener so that the metrics are not exposed
publicly.

roles/synapse/templates/homeserver.yaml.j2

@@ -22,6 +22,10 @@ listeners:
   resources:
   - names: [client, federation]
     compress: false
+- type: metrics
+  port: 9000
+  bind_addresses:
+  - 0.0.0.0
 
 retention:
 
@@ -38,6 +42,7 @@ registration_shared_secret: '{{ synapse_registration_shared_secret }}'
 
 account_threepid_delegates:
 
+enable_metrics: true
 metrics_flags:
 
 report_stats: false