rbd-mirror: add and rename perf counters for journal and snapshot mirroring

rbd-mirror: add and rename perf counters for journal and snapshot mirroring

This commit renames the existing snapshot and journal based performance
counters in the rbd-mirror daemon to better reflect their purpose.
Additionally, new perf counters have been added to provide more detailed
information about the synchronization of snapshots between the source
and destination clusters.

This commit also switches to use labels instead of having the image spec
in the name of the counter.

Signed-off-by: Divyansh Kamboj <dkamboj@redhat.com>