simplify langchain distributed tracing example

[klaasmeinke]

I think that the previous example was complicating the solution for just adding a trace id.

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Important

Simplifies Langchain trace ID handling by using trace_context in CallbackHandler instead of span wrapping.

• Documentation:
  • Simplifies the process of adding a trace ID to Langchain executions in trace-ids-and-distributed-tracing.mdx.
  • Replaces the span wrapping method with a direct trace_context argument in CallbackHandler.
  • Removes unnecessary span creation and updates example code accordingly.

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