Code Review Analytics: Review Time calculation

Summary

Based on #38062 (comment 269477889)

In order to properly sort MRs list on review time we should store "first non-author comment creation date" timestamp, so we can effectively calculate actual review time for any given MR.

Proposal

Fill first_comment_at field in merge_request_metrics table when non-author posts any comment on a MR. Recalculate first_comment_at when comment deletion happens.

Open questions

1. What should we do if user deletes first comment some time later? Should we recalculate for new first_comment_at date or no?
2. What about data migration? Currently first_comment_at field is filled for "merged" MRs only, so there will be no data filled for open MRs leaving us with no data right after CRA deploy. Currently we have about 4.8mln open MRs at gitlab.com OUT OF SCOPE
3. What should happen if a client moves group\instance from free to Starter+ license? Should we trigger data migration to backfill data of all open MRs? OUT OF SCOPE
4. Considering plans to extend list of MRs to merged\closed MRs data migration can have wider scope in future. OUT OF SCOPE