Instrument client API for metrics

In talking about ACCUMULO-2388, we noted that the client implementation doesn't have instrumentation for collecting important metrics. This would have benefit for users trying to debug performance of their applications as well as use for the developers to validate some of our decisions.

We have hadoop metrics2 on the servers right now to fill this gap. I'm not sure what we would want to use for this. Metrics2 might not be the best tool in its current state. I've heard decent things about dropwizard metrics (formerly codahale metrics) in the past. I'm sure there are many we could use.