Improve agent loop observability in Langfuse by including additional metadata in calls to LiteLLM

[mattflo]

The LiteLLM and Langfuse integration support sending some special metadata that allows for better relating and aggregating of trace data in Langfuse:

• https://langfuse.com/docs/integrations/litellm/tracing#set-custom-trace-id-trace-user-id-and-tags
• https://docs.litellm.ai/docs/proxy/logging#logging-metadata-to-langfuse

If LibreChat could include some of these in calls to LiteLLM, that would facilitate much better visibility of agent loop activity.

• conversationId -> session_id
• The messageId of the current or nearest ancestor message with sender='User' -> trace_id
• user -> trace_user_id

There's also support to pass along generation_id, generation_name, and tags, but I'm not sure what values make sense to pass for these.