Cloud SQL does not support multitenancy in same way as Datastore. While Datastore supports multitenancy directly, other Google Cloud services do not. If your multitenant app uses other Google Cloud services, you will need to handle multitenancy manually. You may check
Namespaces for related information.
This discussion group is oriented more towards general opinions, trends, and issues of general nature touching the App Engine. For coding and program architecture such as implementing multitenancy in Cloud SQL, you may be better served in dedicated forums such as stackoverflow, where experienced programmers are within reach and ready to help.