Add label changes as timeline events on incidents

Release notes

Problem to solve

Incident responders currently use labels (in particular scoped labels) to indicate things like status on incidents. For this reason, the adding of scoped labels to incidents could be something we want to consider adding timeline events for, automatically. However, we'll need to figure out a way to do this smartly, to avoid overwhelming the timeline with unimportant label changes.

Proposal

Add scoped labels to the timeline automatically. Since scoped labels are a premium feature, automatically adding scoped labels to the timeline would be a premium feature, as well.

Update: There was no way to separate out scoped from un-scoped labels, so it was an all-or-nothing decision. That's why it was merged behind a feature flag, so we can try it out with our team to see if it's too noisy, before enabling more generally. More discussion in this thread.

Intended users

Feature Usage Metrics

Feature flag

This feature is currently behind a feature flag. See [Feature flag] Rollout of `incident_timeline_ev... (#369416).