[openai] Remove references to openai.api.usage metrics (DataDog#21696)

* [openai] Remove references to openai.api.usage metrics

* Apply suggestions from code review

Co-authored-by: Bryce Eadie <bryce.eadie@datadoghq.com>

* Apply suggestions from code review

Co-authored-by: Bryce Eadie <bryce.eadie@datadoghq.com>

* update costs dashboard

* changelog formatting

* update dashboard

* date format

* reverse changelog order

---------

Co-authored-by: Bryce Eadie <bryce.eadie@datadoghq.com> 3458c29

Author: pull[bot]