* Fix bug in usage metrics that caused a negative count to occur
There were a couple of instances were usage metrics would do the wrong
thing and result in incorrect counts, causing the count to attempt to
decrement below zero and return an error. The usage metrics did not
account for various places where a single transaction could
delete/update/add multiple service instances at once.
We also remove the error when attempting to decrement below zero, and
instead just make sure we do not accidentally underflow the unsigned
integer. This is a more graceful failure than returning an error and not
allowing a transaction to commit.
* Add changelog
Chris Piraino