Use Langchain LCEL + Langserve + run time user_id/session_id

[ravediamond]

Describe the feature or potential improvement

When integrating Lanfuse with LCEL, for example in the context of an api with Langserve, it is not possible to set at runtime the user_id or the session_id.
I propose to check the Langchain configurable if there are variables called user_id or session_id and use them to populate these fields in Langfuse.
This is actually what I do in my LangServe API and it is working great.

What do you think about adding this in official callback for Langchain ?

Additional information

No response

[marcklingen]

Hi @ravediamond, open for contributions on this, this is a great idea!