mgr/telemetry: fix device serial number anonymization

mgr/telemetry: fix device serial number anonymization

- Model names sometimes have underscores in them, hence it is
not safe to split the string <vendor>_<model>_<serial> by '_' and expect
the correct corresponding tuple.
For now we'll take the string after the last '_' as the serial.

- Fixed back_ip to be retrieved from cluster_addrs instead of
public_addrs.

- Omitted the ':' from 'pg_num:' key. This won't break the telemetry
server since it does not extract this key, but calculates it from 'pools'.

Signed-off-by: Yaarit Hatuka <yaarit@redhat.com>
(cherry picked from commit e4d5ece05ec76aea7392ee4e41a34082692f0df6)
Conflicts: ceph-mgr in nautilus is still using python2, we should use python2 compatible syntax in this backport commit.
See https://github.com/ceph/ceph/pull/32926