doc/radosgw: add "persistent_topic_size"

Add "persistent_topic_size" to the list of "Notification Performance
Statistics" in doc/radosgw/notifications.rst.

Fixes: https://tracker.ceph.com/issues/71792

Signed-off-by: Zac Dover <[email protected]>
(cherry picked from commit 5f96ade)

Author: zdover23

doc/radosgw/notifications.rst

@@ -125,6 +125,7 @@ Dump (in JSON format) all pending bucket notifications of a persistent topic by
 Notification Performance Statistics
 -----------------------------------
 
+- ``persistent_topic_size``: queue size in bytes. 
 - ``pubsub_event_triggered``: a running counter of events that have at least one topic associated with them
 - ``pubsub_event_lost``: a running counter of events that had topics associated with them, but that were not pushed to any of the endpoints
 - ``pubsub_push_ok``: a running counter, for all notifications, of events successfully pushed to their endpoints