magma: Expose separate data and key index statistics

Introduce additional statistics for compactions to improve supportability.

Currently, we have only aggregated compaction count stats.
The following stats can be exposed.
Data level compaction count
Seq Index compactions count
key index compaction count

Also individual write bytes to understand where the main write amp is coming from.

It is harder to determine which compactions are contributing any system behavior with the aggregate stats that we have today. We need to expose the additional stats