feat: added remote trace attachment

[edwardpwtsoi]

for solving https://github.com/langfuse/langfuse/issues/7838 added remote trace attachment to langchain callback handler with langfuse_trace_id is provided in the config.metadata

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[!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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[hassiebp]

Thanks a lot for your contribution! Closing this in favor of https://github.com/langfuse/langfuse-python/pull/1419