Reindexing: Add statistics tracking

What does this MR do?

With this change, we start to track statistics for index rebuilds.

This allows us to understand the impact/success of index rebuilds as we go. Plus, we can make smarter choices in terms of which indexes to rebuild: !42967 (merged) makes a random choice here (which is fine for now), later we might want to use these statistics to prefer indexes that have had high levels of bloat before or ones that haven't been rebuilt recently.

This is dependent on !42967 (merged), this MR builds on top of it (I'll resolve the dependency/change target branch once the other one has been merged).

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