qa: enhance labeled perf counters test for cephfs-mirror

qa: enhance labeled perf counters test for cephfs-mirror

Implements checks for labeled perf counters in the appropriate tests.
This patch verifies snaps_synced, snaps_renamed, snaps_deleted and sync_failures metrics are
updated correctly based on the tests.

Fixes: https://tracker.ceph.com/issues/64486
Signed-off-by: Jos Collin <jcollin@redhat.com>
(cherry picked from commit 7b7e5d4221389ce077780450d78a4a1e6a3d0a43)

 Conflicts:
	qa/tasks/cephfs/test_mirroring.py
                Conflicted due to c7c38ba not backported