mgr/prometheus: get osd_objectstore once instead twice.

After merging #25234 & #24821 separetly we get in
get_metadata_and_osd_status() double osd_metadata.get('osd_objectstore') call.

Signed-off-by: Konstantin Shalygin <k0ste@k0ste.ru>
(cherry picked from commit bf3231b)

src/pybind/mgr/prometheus/module.py

@@ -498,14 +498,13 @@ def get_metadata_and_osd_status(self):
                     'osd.{}'.format(id_),
                 ))
 
-            osd_objectstore = osd_metadata.get('osd_objectstore', None)
-            if osd_objectstore == "filestore":
+            if obj_store == "filestore":
             # collect filestore backend device
                 osd_dev_node = osd_metadata.get('backend_filestore_dev_node', None)
             # collect filestore journal device
                 osd_wal_dev_node = osd_metadata.get('osd_journal', '')
                 osd_db_dev_node = ''
-            elif osd_objectstore == "bluestore":
+            elif obj_store == "bluestore":
             # collect bluestore backend device
                 osd_dev_node = osd_metadata.get('bluestore_bdev_dev_node', None)
             # collect bluestore wal backend