mon: PGMonitor: prettify access to pg_stats_t in register_pg

Signed-off-by: Joao Eduardo Luis <joao@redhat.com>
mon: PGMonitor: populate scrub timestamps with 'now' on pg creation

Fixes: #9496

Signed-off-by: Joao Eduardo Luis <joao@redhat.com>
Merge pull request #2663 from ceph/wip-9496

mon: PGMonitor: populate scrub timestamps with 'now' on pg creation

Reviewed-by: Samuel Just <sam.just@inktank.com>