Queue is a shipper-specific wrapper around the bare libbeat queue.

WrappedBatch is a bookkeeping wrapper around a libbeat queue batch, to work around the fact that shipper acknowledgements are per-event while the queue can only track an entire batch at a time.

metricsSource is a wrapper around the libbeat queue interface, exposing only the callback to query the current metrics.

EntryID is a unique ascending id assigned to each entry that goes in the queue, to handle acknowledgments within the shipper and report progress to the client.