Feature request: ability to configure rate limiting (quotas) when using custom authentication

First of all, thanks for your work and for that product!

I am not sure if it is some limitation from GCP or it was just implemented that way.

We are using ESPv2 with Cloud Endpoints on App Engine.
In order to authenticate our users we use custom authentication (https://cloud.google.com/endpoints/docs/openapi/authenticating-users-custom).
Thus, we are able to identify client application which is calling our API.

It would be very handy for us to be able to specify quotas based on such client application.

Thanks!