Add optional flags to provides endpoints

Author: rpbritton

metadata.yaml

@@ -45,17 +45,23 @@ peers:
 provides:
   database:
     interface: mysql_client
+    optional: true
   mysql:
     interface: mysql
+    optional: true
   mysql-root:
     interface: mysql
+    optional: true
   metrics-endpoint:
     interface: prometheus_scrape
+    optional: true
   grafana-dashboard:
     interface: grafana_dashboard
+    optional: true
   replication-offer:
     interface: mysql_async
     limit: 1
+    optional: true
 
 requires:
   certificates: