enhancement(http sink): Add support for end-to-end acknowledgements (#7265)

* Create `EventRef` wrapper for references to event inner types

* Rework MetricsBuffer to use the Metric type internally

* Merge some common bits in http sink tests

* Add tests for retries and failures to http sink

* Add Batch wrapper to store metadata

* Use metadata batch for batch sinks

* Update event metadata in `ServiceSink`

* Add metadata output to `trait HttpSink::encode_event`

* Handle metadata in `BatchSink` wrapper

* Modify http sink to pass along metadata

* Add batch status asserts to all HTTP tests

Signed-off-by: Bruce Guenter <bruce.guenter@datadoghq.com>