doc/radosgw: add "persistent_topic_len"

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

Fixes: https://tracker.ceph.com/issues/71791

Signed-off-by: Zac Dover <[email protected]>
(cherry picked from commit 5743303)

Author: zdover23

doc/radosgw/notifications.rst

@@ -126,6 +126,8 @@ Notification Performance Statistics
 -----------------------------------
 
 - ``persistent_topic_size``: queue size in bytes. 
+- ``persistent_topic_len``: shows how many notifications are currently waiting
+  in the queue
 - ``pubsub_event_triggered``: a running counter of events that have at least one topic associated with them
 - ``pubsub_push_ok``: a running counter, for all notifications, of events successfully pushed to their endpoints
 - ``pubsub_push_fail``: a running counter, for all notifications, of events that failed to be pushed to their endpoints