Tracing integrations: allow for custom trace/run_ids

[marcklingen]

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

When working on LLM applications and tracing them, the trace_id is usually key to later on:

• add additional non-haystack spans to the trace
• record user feedback or other evaluation metrics for the trace
• be able to fetch the single trace via API at a later point
• use distributed tracing to log to the same trace id

Currently, there is no way of customizing the trace_id (at least not when using the Langfuse intgeration).

Describe the solution you'd like

I like the DX of LangChain where a run_id can be set via the run_config and is later on used as a trace_id for tracing. See example here.

I am unsure where this is best set within Haystack, probably as part of the run method.

[vblagoje]

@marcklingen can't we simply use invocation_context in LangfuseConnector and then perhaps we can access these values user provided in run to convert them to tags/markers or whatever is needed to mark the trace?

[vblagoje]

Unless I'm overlooking something one should be able to achieve this by adjusting DefaultSpanHandler from #1313 or perhaps implement a custom handler class and leave it here for others to see as an example of customization