chore(datadog_traces sink): Compute APM stats (#12806)

* +code path for APM stats computation

* +aggregator skeleton

* +placeholders for full sketches

* +switch to msgpack for stats

* +minor adjustment

* +adjust encoding to exactly match intake expectation

* +fix sketch offset

* +fmt

* +adjusments & test

* +address reviews & fix ci

* +make check-event happy

* +side cases & inline doc

* +additional doc

* +additional comments

Co-authored-by: Jesse Szwedko <jesse@szwedko.me>