tools/cephfs_mirror: Adds capability to persist metrics

tools/cephfs_mirror: Adds capability to persist metrics

Adds the capability to persist mirroring metrics.
The metrics are persisted in the omap of the cephfs-mirror
object. Metrics are persisted asynchronously.

Each mirrored directory path stores the corresponding
metrics as the value of a unique omap key representing
the mirrored directory. The omap key is as below.

sync_stat/<fsname>/<peer_uuid>/<mirrored_dir_path>

Fixes: https://tracker.ceph.com/issues/76686
Signed-off-by: Kotresh HR <khiremat@redhat.com>
(cherry picked from commit 9f59332b35259773783bbd38e4f92fa1520d6cbf)