pybind/mgr/prometheus/module.py: defaultdict for num_by_state

pybind/mgr/prometheus/module.py: defaultdict for num_by_state

num_by_state[state] += count in get_pg_status method raises KeyError
if pg state is not in PG_STATES list. PG_STATES should be synced with
osd_types.cc:pg_state_string(). But sometimes it is not. After the
KeyError raise mgr metrics are not available at all.

Fixes: https://tracker.ceph.com/issues/46142
Signed-off-by: Alexander Sushko <alexandrsushko@gmail.com>
(cherry picked from commit 3f7ee9cbd335e4b8686688b79ec6110d73a7390e)

 Conflicts:
	src/pybind/mgr/prometheus/module.py
 - Resolved import conflict