feat: langfuse integration

[jacopo-chevallard]

Description

Small PR to integrate Langfuse tracing

Please include a summary of the changes and the related issue. Please also include relevant motivation and context.

Checklist before requesting a review

Please delete options that are not relevant.

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented hard-to-understand areas
• I have ideally added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

Screenshots (if appropriate):

[StanGirard]

Should we make the callback only used when the API keys are set ?

[jacopo-chevallard]

Should we make the callback only used when the API keys are set ?

I don't that it is necessary, it is dealt with within Langfuse in principle

[jacopo-chevallard]

Should we make the callback only used when the API keys are set ?

I confirm, if any API key is missing, no trace is produced, but no error is raised either