Include endpoint in metrics for ETag caching middleware

What does this MR do?

It adds a custom "tag" / "dimension" - endpoint - to metrics for ETag caching middleware. It will allow us to create a custom dashboard in https://performance.gitlab.net/?orgId=1 that shows breakdown of events by endpoint.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together