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.
Additionally, we maintain two variables to retain the previous values of
this metric.

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Could you explain the significance of 5,000,000 used in this line (and others)? It should be pulled out into a value with a meaningful name rather than hardcoded in multiple places.