sorry for missing your note (Haven't seen it, must be moved to a folder, my mailbox has been a bit messy recently, sorry again :)), the quota most of the time is per project but you can for sure do a limit to the quota of the CloudEndpoint per user but it would be using the user defined metrics and in the end it would be on the level of project and per minute, so you define a user defined metrics and in it group it by the user (inquire it from the token) and then the quota will be defined using the user defined quota but again per minute per project.
I am attaching a link to an nice article talking about the different strategies you can implement for rate limiting
I am not sure we should plan to have the quota per user, could be in future per environ or tenant but not sure that we will have a quota defined per user.
Maybe you can share a bit on the needs to have API quotas per user; I am sure it will help clarifying things :).
Sorry again for the so late response, I am really sorry :).