CA-147261: Add average queue size metric for tapdisk3

Prior to this patch, the average queue size metric was missing.
This patch computes this metric using the stats exposed by tapdisk3.
Average queue size is defined as the sum of the average read and write queues.

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Merge pull request #1965 from siddharthv/CA-147261

CA-147261: Add average queue size metric for tapdisk3