adk align Google usage metadata token metrics #638

Problem

The Python ADK integration currently maps Google UsageMetadata as:

• prompt_tokens = prompt_token_count
• completion_tokens = candidates_token_count
• completion_reasoning_tokens = thoughts_token_count
• tokens = total_token_count

This leaves thoughts_token_count out of the completion total and ignores tool_use_prompt_token_count. For reasoning or tool-using calls, the emitted prompt/completion breakdown can therefore disagree with the provider total and with Braintrust instrumentation semantics.

Current implementation: py/src/braintrust/integrations/adk/tracing.py

Provider reference: Google GenerateContent UsageMetadata

Expected behavior

Align ADK usage extraction with the Python Google GenAI integration:

• prompt_tokens = prompt_token_count + (tool_use_prompt_token_count or 0)
• completion_tokens = candidates_token_count + (thoughts_token_count or 0)
• retain completion_reasoning_tokens = thoughts_token_count as the reasoning breakdown
• retain tokens = total_token_count when Google reports it
• preserve any useful provider-specific modality/detail breakdowns as metadata rather than adding nonstandard metrics
• preserve zero values and omit values Google did not report

Acceptance criteria

• Add VCR-backed ADK coverage using real Google response shapes for reasoning and tool-use prompt usage.
• Assert the aggregate prompt/completion metrics and the reasoning detail metric.
• Assert tokens == prompt_tokens + completion_tokens when all components are reported.
• Validate every configured google-adk version in the Python provider matrix, accounting for version-specific field availability.

This is a Python ADK integration change; no instrumentation-spec update should be needed because all emitted metric keys are already standard.