[google-genai] Add gen_ai.usage.reasoning.output_tokens span attribute

[Rima-ag]

Description

Add gen_ai.usage.reasoning.output_tokens span attribute to Google genai instrumentation.

This is still pending merging: #4206 that includes thinking tokens in gen_ai.usage.output_tokens.

Fixes #3890
Related to open-telemetry/semantic-conventions#3194 and #4159

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Unit Tests

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

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

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