Add prometheus gauge for the online GC review queues size

Sample:

# HELP registry_gc_queue_size The size of online GC review queues.
# TYPE registry_gc_queue_size gauge
registry_gc_queue_size{queue="gc_blob_review_queue"} 12
registry_gc_queue_size{queue="gc_manifest_review_queue"} 10

Related to #293 (closed)