Update usage ping data to track usage of DiffNotes

Problem

The performance indicator for groupsource code is MR usage. Users who comment on diffs are considered MR users, however, this is not currently tracked by usage ping.

Proposal

Add DiffNote usage to usage ping. This should help us answer the question: How many unique users have used DiffNotes?

Links

PI issue https://gitlab.com/gitlab-data/analytics/-/issues/4633