Fix matrix.DOMAIN/_synapse/metrics exposing

This is something that got lost during
#456
and more specifically 4d62a75.

Fixes #914

roles/matrix-nginx-proxy/defaults/main.yml

@@ -193,6 +193,8 @@ matrix_nginx_proxy_proxy_matrix_client_api_forwarded_location_prefix_regexes: |
     (['/_synapse/oidc'] if matrix_nginx_proxy_proxy_matrix_client_api_forwarded_location_synapse_oidc_api_enabled else [])
     +
     (['/_synapse/admin'] if matrix_nginx_proxy_proxy_matrix_client_api_forwarded_location_synapse_admin_api_enabled else [])
+    +
+    (['/_synapse/metrics'] if matrix_nginx_proxy_proxy_synapse_metrics else [])
   }}
 
 # Specifies where requests for the root URI (`/`) on the `matrix.` domain should be redirected.