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>
@franciozzy I've already done a very similar test and the metrics from `xcp-rrdd` and `xsiostat` are analogous.