mgr/DaemonServer: fetch metadata for new daemons (e.g., mons)

mgr/DaemonServer: fetch metadata for new daemons (e.g., mons)

We fetch metadata for mon/osd/mds daemons on startup.  If a new one comes
along *after* we start up, we need to fetch it on demand.  Otherwise,
notably, we will ignore any new mons added to the cluster:

- mon.a starts
- mgr starts, fetchs a's metadata
- mon.b added
- mon.b tries top open mgr connection, and is rejected each time

This can lead to follow-on badness when the mon tries to proxy mgr
commands or do other things.

Fix by fetching metadata on demand, like we already do in the report
path.

Fixes: https://tracker.ceph.com/issues/44798
Signed-off-by: Sage Weil <sage@redhat.com>