Instrument agent logging

Logging internals instrumentation.  @peterbourgon @bernerdschaefer 
> That said, it's entirely possible I've missed some Best Practice memo, please feel free to educate me...

We're going to be monitoring the health of our entire infrastructure through these counters.  They better be counting what we think the say they are counting, and doing it when we think they should be doing it.

[One of these tests (actually, the very last one I wrote) did show that I'd screwed up where I put one of the counter statements.]

> seems needless—like testing log output.

I don't think you realize how many weeks my life at YouTube was consumed cleaning up messes caused by other developers changing logging output with the expectation that logging didn't matter, or by subtly changing the conditions under which logging messages were sent.