Usage metadata extraction from serialized tracer outputs now supported

langchain-core 1.2.17 fixes extraction of usage metadata from serialized tracer message outputs, ensuring token usage and cost tracking work correctly when traces are serialized/deserialized. Previously, usage metadata could be lost during serialization round-trips.

Verify that langchain_tokens_prompt, langchain_tokens_completion, and langchain_llm_cost metrics are correctly populated after upgrading. Check LangSmith traces for complete usage metadata in serialized runs.