Add matrix_nginx_proxy_proxy_synapse_metrics_addr_{with,sans}_container variables to defaults

We redefine these variables in `group_vars/matrix_servers`, but it's better
to have some defaults in the role as well.

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

@@ -225,6 +225,8 @@ matrix_nginx_proxy_proxy_synapse_metrics_basic_auth_enabled: false
 # The part after `prometheus:` is needed here. matrix_nginx_proxy_proxy_synapse_metrics_basic_auth_key: "$apr1$wZhqsn.U$7LC3kMmjUbjNAZjyMyvYv/"
 matrix_nginx_proxy_proxy_synapse_metrics_basic_auth_key: ""
 matrix_nginx_proxy_proxy_synapse_metrics_basic_auth_path: "{{ matrix_nginx_proxy_data_path_in_container if matrix_nginx_proxy_enabled else matrix_nginx_proxy_data_path }}/matrix-synapse-metrics-htpasswd"
+matrix_nginx_proxy_proxy_synapse_metrics_addr_with_container: "matrix-synapse:9100"
+matrix_nginx_proxy_proxy_synapse_metrics_addr_sans_container: "127.0.0.1:9100"
 
 # The addresses where the Matrix Client API is.
 # Certain extensions (like matrix-corporal) may override this in order to capture all traffic.