Add schema for "metrics" object in Ambari responses

Add a schema for the "metrics" object in the responses for /clusters/<name>/<node>/host_components/<component_name> and clusters/<cluster_name>/services/<service_name>/components/<component_name>. The current schemas (https://github.com/apache/ambari/blob/trunk/ambari-server/docs/api/v1/schemas.md) list the "metrics" object as optional (good), but do not specify a schema for the object. Its schema should include metrics (each as optional) available via JMX.