mon/MgrStatMonitor: ignore MMgrReport from non-active mgr

mon/MgrStatMonitor: ignore MMgrReport from non-active mgr

If it's not the active mgr, we should ignore it.

Since the mgr instance is best identified by the gid, add that to the
message.  (We can't use the source_addrs for the message since that is
the MgrStandby monc addr, not the active mgr addrs in the MgrMap.)

This fixes a problem where a just-demoted mgr report gets processed and a
new mgr gets a ServiceMap with an epoch >= its pending map.  (At least,
that is my theory!)

Fixes: https://tracker.ceph.com/issues/48022
Signed-off-by: Sage Weil <sage@newdream.net>