mgr/progress: avoid inefficient dump of all pg stats

mgr/progress: avoid inefficient dump of all pg stats

We only use a handful of fields, and the pg dump includes a gazillion
fields that we waste CPU copying to python-land.  This tends to lead to
long ClusterState::lock hold times, leading to long ms_dispatch delays
and generally gumming up the works.

Instead, create a new "pg_progress" item that dumps only the fields that
mgr/progress needs.

Fixes: https://tracker.ceph.com/issues/53475
Signed-off-by: Sage Weil <sage@newdream.net>