add vb_avg_total_queue_age to histograms
add vb_avg_total_queue_age to histograms

Change-Id: I150289f4aa4068dc5d43144425b72300ef592d38
Reviewed-on: http://review.couchbase.org/c/cbmonitor/+/142649
Tested-by: Bo-Chun Wang <bb86fd206c9c35341feb9b50e74a74a83e254d63@couchbase.com>
Reviewed-by: Bo-Chun Wang <bb86fd206c9c35341feb9b50e74a74a83e254d63@couchbase.com>

couchbase/cbmonitor,couchbase/cbmonitor