observability: add metrics so we can measure frequency and duration of PVC snapshot backup and restore

[sagor999]

Frequency:

• PVC snapshot backup
  • workspace_backups_success_total.
  • workspace_backups_failure_total.
• PVC snapshot restore
  • workspace_restores_success_total.
  • workspace_restores_failure_total.

Duration:

• PVC snapshot backup: gitpod_ws_manager_volume_snapshot_seconds.
• PVC snapshot restore: gitpod_ws_manager_volume_restore_seconds.

[sagor999]

@jenting assigning to you, similar to this: #9355

[jenting]

For the PVC snapshot restore time, we could calculate the time of the PVC object state from initialized to bound.