Important

Adds remote trace attachment to LangchainCallbackHandler in CallbackHandler.py when langfuse_trace_id is provided in metadata.

• Behavior:
  • Adds remote trace attachment in LangchainCallbackHandler when langfuse_trace_id is in metadata and parent_run_id is None.
  • Modifies on_chain_start() to create a new observation with trace_context if langfuse_trace_id is provided.
• Misc:
  • Addresses issue #7838 on GitHub.

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