Instrument agent logging

Logging internals instrumentation.  @peterbourgon @bernerdschaefer 
I'm willing to be convinced otherwise, but I don't see any reason to test instrumentation. We can safely assume a `prometheus.Counter.Add` invocation works, and testing that a certain action yields increments to counters A, B, and C seems needless—like testing log output.

That said, it's entirely possible I've missed some Best Practice memo, please feel free to educate me...