Best practice for storing secret in app.yaml

I'm deploying a Node.js app on GAE that connects to a Cloud SQL.

Following the docs, I'm told to store the user/password for the database inside app.yaml:

env_variables:
  MYSQL_USER: YOUR_USER
  MYSQL_PASSWORD: YOUR_PASSWORD
  MYSQL_DATABASE: YOUR_DATABASE
  # e.g. my-awesome-project:us-central1:my-cloud-sql-instance
  INSTANCE_CONNECTION_NAME: YOUR_INSTANCE_CONNECTION_NAME

Is this really a good place to store the password?

via G. Alon