Follow up from pull request #3400 for Google Gen AI code

[michaelsafyan]

The pull request https://github.com/open-telemetry/opentelemetry-python-contrib/pull/3400/files was merged with some unaddressed items for Google Gen AI:

1. Correctly document metrics as supported
2. Use a single source of truth for the version of google-genai that is instrumented

[michaelsafyan]

Please assign to me (I'm not able to edit assignees). Created this for tracking purposes to not forget.

[emdneto]

@michaelsafyan I would also add that google-genai package is missing the trove classifier for Python 3.8, 3.9 and 3.13 in pyproject.toml

[michaelsafyan]

Thanks. Will look into it.

[xrmx]

FYI you have already two sources of truth in the code, in instrumentor.py please use _instruments from package.py