template metric toggle for mautrix bridges supporting it

roles/custom/matrix-bridge-mautrix-facebook/defaults/main.yml

@@ -46,6 +46,9 @@ matrix_mautrix_facebook_homeserver_token: ''
 # If false, created portal rooms will never be federated.
 matrix_mautrix_facebook_federate_rooms: true
 
+# Whether or not metrics endpoint should be enabled.
+matrix_mautrix_facebook_metrics_enabled: false
+
 matrix_mautrix_facebook_bridge_permissions: |
   {{
     {matrix_mautrix_facebook_homeserver_domain: 'user'}