mds: use regular dispatch for processing metrics

mds: use regular dispatch for processing metrics

There have been cases where the MDS does an undesirable failover because it
misses heartbeat resets after a long recovery in up:replay.  It was observed
that the MDS was processing a flood of metrics messages from all reconnecting
clients. This likely caused undersiable MetricAggregator::lock contention in
the messenger threads while fast dispatching client metrics.

Instead, use the normal dispatch where acquiring locks is okay to do.

See-also: linux.git/f7c2f4f6ce16fb58f7d024f3e1b40023c4b43ff9
Fixes: https://tracker.ceph.com/issues/65658
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>