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.