[ceph-fs] Ceph RBD/FS top

Hey Jason, We are working towards bringing in `top` like functionality to CephFS for displaying various client (and MDS) metrics. Since RBD has something similar in the form of `perf image io*` via rbd cli, we would like to understand some finer details regarding its implementation and detail how CephFS is going forward for `fs top` functionality. IIUC, the `rbd_support` manager module requests object perf counters from the OSD, thereby extracting image names from the returned list of hot objects. I'm guessing it's done this way since there is no RBD related active daemon to forward metrics data to the manager? OTOH, `rbd-mirror` does make use of `MgrClient::service_daemon_update_status()` to forward mirror daemon status, which seems to be ok for anything that's not too bulky. For forwarding CephFS related metrics to Ceph Manager, sticking in blobs of metrics data in daemon status doesn't look clean (although it might work). Therefore, for CephFS, `MMgrReport` message type is expanded to include metrics data as part of its report update process, as per: https://github.com/ceph/ceph/pull/26004/commits/a75570c0e73ef67bbca8f73a974… ... and a callback function is provided to `MgrClient` (invoked periodically) to fill in appropriate metrics data in its report. This works well and is similar to how OSD updates PG stats to Ceph Manager. I guess changes of this nature was not required by RBD as it can get the required data by querying the OSDs (and were other approaches considered regarding the same)? Thanks, -Venky