Follow-up from "Remove "category" from known events"

The following discussion from !110834 (merged) should be addressed:

• @marc_shaw started a discussion: (+3 comments)

  I am a little behind on the newer aggregation work.

  Taking i_code_review_mr_diffs as an example, this is actually per MR, and not per user, and the idea of this test is that we only aggregate the user metrics, and not mixing both MR and User.

  I see the i_code_review_mr_diffs metric in here https://gitlab.com/gitlab-org/gitlab/-/blob/master/config/metrics/counts_7d/20210216184452_code_review_total_unique_counts_weekly.yml#L78, but at the top, we have https://gitlab.com/gitlab-org/gitlab/-/blob/master/config/metrics/counts_7d/20210216184452_code_review_total_unique_counts_weekly.yml#L17

  I don't see how it knows that i_code_review_mr_diffs is a merge request and not a user id.