osd: log the number of 'dups' entries in a PG Log

osd: log the number of 'dups' entries in a PG Log

We really want to have the ability to know how many
entries `PGLog::IndexedLog::dups` has inside.
The current ways are either invasive (stopping an OSD)
or indirect (examination of `dump_mempools`).

The code comes from Nitzan Mordechai (part of
ede37edd79a9d5560dfb417ec176327edfc0e4a3).

Fixes: https://tracker.ceph.com/issues/55982
Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>
(cherry picked from commit 8f1c8a7309976098644bb978d2c1095089522846)