Merge https://github.com/google/adk-python/pull/2109Fixes#2105
## Problem
When integrating Google ADK with Langfuse using the @observe
decorator, the usage details displayed in Langfuse web UI were
incorrect.
The root cause was in the telemetry implementation where
total_token_count was being mapped to gen_ai.usage.output_tokens
instead of candidates_token_count.
- Expected mapping:
- candidates_token_count → completion_tokens (output tokens)
- prompt_token_count → prompt_tokens (input tokens)
- Previous incorrect mapping:
- total_token_count → completion_tokens (wrong!)
- prompt_token_count → prompt_tokens (correct)
## Solution
Updated trace_call_llm function in telemetry.py to use
candidates_token_count for output token tracking instead of
total_token_count, ensuring proper token count reporting to
observability tools like Langfuse.
## Testing plan
- Updated test expectations in test_telemetry.py
- Verified telemetry tests pass
- Manual verification with Langfuse integration
## Screenshots
**Before**
<img width="1187" height="329" alt="Screenshot from 2025-07-22 20-20-33" src="https://github.com/user-attachments/assets/ad5fc957-64a2-4524-bd31-0cebb15a5270" />
**After**
<img width="1187" height="329" alt="Screenshot from 2025-07-22 20-21-40" src="https://github.com/user-attachments/assets/3920df2a-be75-47e0-9bd0-f961bb72c838" />
_Notes_: From the screenshot, there's another problem: thoughts_token_count field is not mapped, but this should be another issue imo
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/2109 from tl-nguyen:fix-telemetry-token-count-mapping 3d043f558b5f8bcb2c6e0370e2cc4c0ff25d1f4a
PiperOrigin-RevId: 786827802
