This is just a warning caused by the server process of Cloud SQL and it does not affect your Cloud SQL for MySQL instance. The warning is removed now and will be released in next upcoming release.
Please follow below steps to configure the exclusion filter, if you wish to suppress it :
Navigate to the Cloud Logging from the Cloud Console.
Click on Logs Router (On the left side of the Cloud Logging page)
Click on the settings of the default bucket(3 dots)
Click on Edit
Click on Add exclusion
Add an exclusion filter to match the warning and update the sink. This will not persist the warning configured and the log looks tidy.