(Incident 19118) Add alerting to detect broken diff comment rendering

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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.