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Fixes missing `run.metadata.usage_metadata` population in `LangChainTracer` for real LLM/chat traces following #34414 - Fix extraction to read usage from serialized tracer message shape: `outputs.generations[*][*].message.kwargs.usage_metadata` - Remove non-serialized direct message shape handling (`message.usage_metadata`) from extractor to match real tracer output path - Clarify tracer docstrings around chat callback naming (`on_chat_model_start` + shared `on_llm_end`) to reduce ambiguity ## Why #34414 introduced usage duplication into `run.metadata.usage_metadata`, but the extractor read `message.usage_metadata`. In real tracer flow, messages are serialized with `dumpd(...)` during run completion, so usage metadata lives under `message.kwargs.usage_metadata`. Because of this mismatch, duplication did not trigger in real traces.