Add metrics of workers per queue
e.g.
celery_queue_worker{queue_name="cleanup"} 1.0
celery_queue_worker{queue_name="pungi_composes"} 2.0
celery_queue_worker{queue_name="pulp_composes"} 0.0
JIRA: RHELCMP-11505
Signed-off-by: Haibo Lin <hlin@redhat.com>