(PDB-3830) Decrement queue depth when discarding a message
(PDB-3830) Decrement queue depth when discarding a message

Prior to this commit, when a message is discarded it had no effect
on the command queue depth metrics.

After this commit, when a message is discarded we decrement the
queue depth metric.

rbrw/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb,puppetlabs/puppetdb,rbrw/puppetdb,rbrw/puppetdb,rbrw/puppetdb,rbrw/puppetdb,puppetlabs/puppetdb