Add metrics for snapshots

[hafe]

Is your feature request related to a problem? Please describe

As discussed in Slack, metrics are missing related to snapshots.
One use case is to monitor that backups happen. The documented backup process creates a partition snapshot via /internal/partition/snapshot/create, copies it with rsync or rclone and then deletes it. Currently this activity is visible only in VictoriaLogs logs.

Describe the solution you'd like

Snapshot related metrics (creates, deletes) returned by the normal metrics endpoint. For example:

• the current number of snapshots, similar to the vm_snapshots metric in VictoriaMetrics. This would also help detect snapshots, which weren't removed and occupy disk space.
• the number of created and deleted snapshots, so it is possible to alert when backups didn't happen for some time.