fix: merge resource attributes into existing tracers

[LucasAlvesSoares]

Description

In auto_instrument, access private "tracer" attributes in order to merge runtime resource attributes into existing trace providers, e.g K8S OTel operator.

Type of Change

Please check the relevant option:

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactoring
• Dependency update

How to Test

1. Run auto_instrument() on an agent
2. Trigger an LLM call so that it emits telemetry
3. Resource attributes should look exactly the same across all spans

Checklist

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• I have read the Contributing Guidelines
• I have verified that my changes solve the issue
• I have added/updated automated tests to cover my changes
• All tests pass locally
• I have verified that my code follows the Code Guidelines
• I have updated documentation (if applicable)
• I have added type hints for all public APIs
• My code does not contain sensitive information (credentials, tokens, etc.)
• I have followed Conventional Commits for commit messages