The mysql performance_schema method for counting per-database queries
is very heavyweight in that it requires full logging (in a table) of
every query. We do hundreds of thousands in the course of a tempest
run, which ends up creating its own performance problem.
This changes the approach we take, which is to bundle a very tiny
sqlalchemy plugin module which counts just what we care about in
a special database.
It is more complex than just enabling the features in mysql, but it
is a massively smaller runtime overhead. It also provides us the
opportunity to easily zero the counters just before a tempest run.
Change-Id: I361bc30bb970cdaf18b966951f217862d302f0b9
fe52d7f0a8