Detailed backup stats

[maxenglander]

Description

I would like to have better instrumentation on backups, in particular backups generated by vtbackup. While backup stats are exposed via servenv since #11388 (which is great!) ideally I would like more fine-grained stats on:

• Download last backup.
• Decompress last backup.
• Catchup to primary with replication.
• Compress new backup.
• Upload new backup.

Use case

I want to make backups (and restores) as fast as possible. Having more fine-grained stats will guide me to put energies in the right place.

[maxenglander]

There are other backup plugins that would benefit from detailed stats, but think this is far enough along that we can close this out.