chore(llmobs): token metrics name changes (#9657)

Makes the following updates to metric key names submitted to LLM
Observability for openai & bedrock integrations

`prompt_tokens` -> `input_tokens`
`completion_tokens` -> `output_tokens`

The backend already has the changes in place to accept these updated key
names so a hard cutover is OK.

A release note is not needed since metric key names used by our
integrations (openai, langchain, bedrock) when submitting data to LLM
Obs backend is an internal contract between the integration and backend.

When users set metric key names for manually created spans, our
documentation already instructs them to use input/output terminology

- [x] Change(s) are motivated and described in the PR description
- [x] Testing strategy is described if automated tests are not included
in the PR
- [x] Risks are described (performance impact, potential for breakage,
maintainability)
- [x] Change is maintainable (easy to change, telemetry, documentation)
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
are followed or label `changelog/no-changelog` is set
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/))
- [x] Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))
- [x] If this PR changes the public interface, I've notified
`@DataDog/apm-tees`.

- [x] Title is accurate
- [x] All changes are related to the pull request's stated goal
- [x] Description motivates each change
- [x] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- [x] Testing strategy adequately addresses listed risks
- [x] Change is maintainable (easy to change, telemetry, documentation)
- [x] Release note makes sense to a user of the library
- [x] Author has acknowledged and discussed the performance implications
of this PR as reported in the benchmarks PR comment
- [x] Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

---------

Co-authored-by: lievan <evan.li@datadoqhq.com>
Co-authored-by: kyle <kyle@verhoog.ca>