feat(client): allow more params.(like langfuse_version, etc)

[XenoAmess]

[!IMPORTANT] Enhance on_chain_start in langchain.py to dynamically update trace-level parameters using metadata prefixed with langfuse_.

• Behavior:
  • In on_chain_start in langchain.py, trace-level information is updated with metadata keys prefixed by langfuse_.
  • This allows dynamic updates to parameters like session_id and user_id based on metadata.
• Misc:
  • Refactored code to use a dictionary update_dict for updating trace information.

^{This description was created by for 0b4f495ae1c9144f0d249718c819b146c08cb5a2. It will automatically update as commits are pushed.}

Greptile Summary

Disclaimer: Experimental PR review

Added support for dynamic trace parameter updates through metadata prefixing in the langchain callback handler, but introduced a bug that breaks existing functionality.

• Fixed bug in on_chain_start where session_id and user_id are incorrectly set to None instead of using metadata values
• Added support for dynamic parameter updates via langfuse_ prefixed metadata keys
• Improved code organization by collecting updates in a dictionary before applying to trace
• Maintained backward compatibility for existing metadata parameter handling

The main issue is that the current implementation always sets session_id and user_id to None in the update_dict, overriding any values that may have been set in metadata. This needs to be fixed to preserve the existing functionality while adding support for the new dynamic parameters.

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[hassiebp]

@XenoAmess Thanks for raising this! As per our contributing guide, could you please open a Github issue / discussion first to lay out your line of thinking for this change to continue the discussion there on why this should be merged / this feature is necessary?

[XenoAmess]

@XenoAmess Thanks for raising this! As per our contributing guide, could you please open a Github issue / discussion first to lay out your line of thinking for this change to continue the discussion there on why this should be merged / this feature is necessary?

sure, at https://github.com/orgs/langfuse/discussions/6697

[hassiebp]

Thanks for raising this @XenoAmess - please upgrade to the Langfuse SDK v3 as the v2 will only get critical bug fixes and security patches