add grafana and alertmanager to metrics network

ansible/plays/services/mimir/docker-compose.yaml

@@ -37,8 +37,9 @@ services:
       - ./alertmanager.yml:/etc/alertmanager/config.yml:ro,Z
       - alertmanager_data:/data
     networks:
-      - backend
       - alertmanager
+      - backend
+      - metrics
       - default
     restart: unless-stopped
     command:
@@ -77,4 +78,6 @@ networks:
     external: true
   backend:
     internal: true
+  metrics:
+    external: true
   alertmanager: