Improve Marginalia comments for API requests

This change adds a `endpoint_id` annotation to the Marginalia comments
to help trace SQL queries to their origin. Examples:

* For Grape API requests, this is the route
  (e.g. `/api/:version/users/:id`).
* For GraphQL requests, this is `GraphqlController#execute`.
* For Rails requests, this is in the controller and action
  (e.g. `Projects::BlobController#show`).

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/323161