(Incident 19118) Add alerting to detect broken diff comment rendering
<!--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=515000)
</details>
<!--IssueSummary end-->
## Context
Follow-up to https://gitlab.com/gitlab-org/gitlab/-/issues/511302 / https://gitlab.com/gitlab-com/gl-infra/production/-/issues/19118.
Idea brought up in https://gitlab.com/gitlab-com/gl-infra/production/-/issues/19121#note_2310339653.
## Proposal
1. Alerting on diff comment viewing anomalies - would be best.
2. Alerting on keep-around refs trends - tricky because there's an active Working Group working on deleting them. Might lead to false negatives, but would raise awareness which is good.
issue