mgr: fix crash due to multiple sessions from daemons with same name

mgr: fix crash due to multiple sessions from daemons with same name

Don't assume perf counter instances are stable. If there are
multiple sessions from daemons reporting the same name (e.g. rgws),
the instances are cleared in DaemonServer::handle_open.
If the daemons have different counter names (for counters like
"objecter-0x55b6f4f1a630.op_active") it will throw out_of_range in
DaemonPerfCounters::update when accessing the instance removed by
another session.

The regression was introduced when adding avgcount support.

Fixes: https://tracker.ceph.com/issues/36244
Signed-off-by: Mykola Golub <mgolub@suse.com>