Bind metrics on :: too

For consistency with all our other listeners,
we make this one bind on the `::` address too
(both IPv4 and IPv6).

Additional details are in #91 (Github Pull Request).

roles/matrix-synapse/templates/synapse/homeserver.yaml.j2

@@ -152,8 +152,7 @@ listeners:
 {% if matrix_synapse_metrics_enabled %}
   - type: metrics
     port: {{ matrix_synapse_metrics_port }}
-    bind_addresses:
-      - '0.0.0.0'
+    bind_addresses: ['::']
 {% endif %}
 
 {% if matrix_synapse_federation_enabled and matrix_synapse_tls_federation_listener_enabled %}