Add optional flags to provides endpoints

Author: rpbritton

metadata.yaml

@@ -24,10 +24,13 @@ containers:
 provides:
   database:
     interface: mysql_client
+    optional: true
   metrics-endpoint:
     interface: prometheus_scrape
+    optional: true
   grafana-dashboard:
     interface: grafana_dashboard
+    optional: true
 requires:
   backend-database:
     interface: mysql_client