Missing in documentation, example of creation secrets for OAUTH2

I am trying to setup Oauth2 with Google provider. In documentation the example showed with hardcoded secrets which is not appropriate way to provide secrets. Also I have used the existingSecret with AUTH_OAUTH2_CLIENT_GOOGLE_CLIENTID and AUTH_OAUTH2_CLIENT_GOOGLE_CLIENTSECRET fields but still it does not help, please provide documented way for this.