mgr/stats: change in structure of perf_stats o/p

mgr/stats: change in structure of perf_stats o/p

*Adding fs_name as a field option in perf_stats o/p
*perf stats command incorrect output with non-existing mds_rank filter
 If `ceph fs perf stats` runs with non-existing mds_rank filter,
 even then it shows all the clients `client_metadata` and `global_metrics`

Fixes: https://tracker.ceph.com/issues/56162
Fixes: https://tracker.ceph.com/issues/56169
Signed-off-by: Neeraj Pratap Singh <neesingh@redhat.com>