MB-7435: Add tap producer stats that shows num of sent items per vb

Change-Id: Iccb666bdaf85d628305925b61a2a9e0cc975570c
Reviewed-on: http://review.couchbase.org/25243
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>