Langfuse Integration: expose the trace_id in the response

[marcklingen]

Is your feature request related to a problem? Please describe.

The trace_id is helpful to add evaluation metrics to the haystack trace within Langfuse. Currently, it needs to be parsed from the trace_url. Exposing the id directly would be a better DX.

Describe the solution you'd like

response = pipe.run(
    data={"prompt_builder": {"template_variables": {"location": "Berlin"}, "prompt_source": messages}}
)
 
trace_id = response["tracer"]["trace_id"]