"... and I don't see if I would be able to track usage from individual end-users through a header".

Currently we have a unified concept of User-Paths. Once you add a specific header OR assign User-Path to an API key, you can track the usage based on this. The User-Path might be youe end-user, internal user or some service. Examples:

  /client1/app1
  /agents/agent1
  /team2/john
  /team2/adam

Would this work for you?

https://gomodel.enterpilot.io/docs/features/user-path

PS Thanks for the feedback on the Vault integration. Noted.