Add a counter for the number of bytes transmitted by the shipper

[cmacknz]

Each time the Elasticsearch output successfully writes a batch to Elasticsearch, we should increment a counter with the number of bytes that were indexed. This counter can be used to derive the shipper transmission rate in bytes/second.

Acceptance Criteria:

• A transmitted bytes metric counter is available on the shipper's metrics HTTP interface under the /debug/vars path.
• A test exists proving that the counter increments when a batch is successfully written and does not increment when the batch fails and needs to be retried.

[pierrehilbert]

@cmacknz / @leehinman could we close this one with the new shipper plan? Because it's already available in beats.

[cmacknz]

Let's leave this open and then confirm Beats works the way we want it to here.