Add client and server instrumentation

[film42]

Server metrics:

1. Message received.
2. Message dropped.
3. Thread pool execution delay.
4. Request duration.

Client metrics:

1. Request duration.
2. A nack.
3. An ack timeout.

Let me know if you have ideas for better names or how to make some of these tests look nicer.

Next is to add a tiny companion gem to wire these up to harness.

cc @abrandoned @mmmries

[abrandoned]

:shipit:

[film42]

Woot! Created https://github.com/mxenabled/harness-protobuf-nats to auto send these metrics to statsd via harness.