Grafana annotations for database reindexing

What does this MR do?

This adds the ability to have Grafana annotations for database reindexing actions. Currently, this targets GitLab.com only - database reindexing feature is still behind a feature flag.

Related docs change (I'll send this one after we merge here): !50369 (merged)

Relates to #273198 (closed)

Screenshots (strongly suggested)

(Local Grafana with example data + reindexing annotations)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content