e2e: Test EPP Metrics Endpoint

[danehans]

Add an e2e test that:

1. Exposes EPP metrics based on this guide.
2. Generates traffic through the EPP to the backend LLM.
3. Scrapes the metrics supported by EPP.

[danehans]

@delavet since you recently added an e2e test, is this something you can take on?

[delavet]

Sure, glad to help👍
/assign