mds: track client provided metric flags in session
With PR https://github.com/ceph/ceph/pull/26004, MDS will start tracking client provided metrics. However, the set of metrics forwarded by kclient (https://tracker.ceph.com/issues/43215) and ceph-fuse (+ libcephfs) will not be the same since it is much harder in the kernel client to track certain metrics. Therefore, clients would need to provide their "metric capabilities" during establishing a session with the MDS. MDS can tag such "missing" metrics as "unavailable".