<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=450957) </details> <!--IssueSummary end--> We should log the `mr-diffs-mark-first-diff-file-shown` event on production (gitlab.com) and store it for a later use in Grafana, so that we can track down the Time to first diff metric.