Expose metric to track index rebuilds through prometheus

In order to visualize status and impact of reindexing, we are going to expose prometheus metrics for that.

This can be based on the activity and results tracking, that is already implemented. For example, we capture when we start a reindexing action and we also capture the end and the result of it.

• Allow observing the current state of reindexing (what is currently running?) and the history of events (when did we see reindexing actions?)
• We may want to report impact of reindexing (size reduction of indexes) over time, too.