Add new cached token gen ai attribute to Google gen AI instrumentation

[DylanRussell]

Description

Sets the new gen_ai.usage.cache_read.input_tokens attribute on spans / events generated by the Google gen AI instrumentation when the experimental sem conv flag is set.. Also does some minor code cleanup..

I don't think the gen_ai.usage.cache_creation.input_tokens attribute makes sense for Gemini.. You can explicitly set a cache but there's no number of tokens associated with that: https://ai.google.dev/gemini-api/docs/caching#explicit-caching

Note that the sem conv says to include cached token in the input token count, which I believe we are already doing without this PR because we use prompt token count which IIUC is the total input tokens based on https://ai.google.dev/gemini-api/docs/tokens#count-tokens and https://github.com/googleapis/python-genai/blob/1fdb4b87aaec6e58b415168ea5893c0e901819a9/google/genai/types.py#L7306C4-L7307C1

Fixes #4159

Type of change

Please delete options that are not relevant.

• [ x] New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Unit tests

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• [x ] No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• [ x] Followed the style guidelines of this project
• [ x] Changelogs have been updated
• [ x] Unit tests have been added
• [ x] Documentation has been updated

[github-actions]

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[tammy-baylis-swi]

Overall lgtm

[github-actions]

This PR has been automatically marked as stale because it has not had any activity for 14 days. It will be closed if no further activity occurs within 14 days of this comment.
If you're still working on this, please add a comment or push new commits.