DatadogBackend attaches custom attributes to wrong span

[timmc-edx]

When we record custom attributes, the intent is to record them on the span representing the current Django request, Celery task, or other service-entry span. Our use of ddtrace.tracer.current_root_span() was intended to implement that, but it turns out this can return a span from outside the current service/process if the trace started in another service.

This could mean that if service A calls service B, attributes meant for the Django request in B are attached to the one in A, possibly overwriting values there.

Acceptance criteria:

• DatadogBackend.set_attribute attaches to the service entry span, not the trace root span

Implementation notes:

• Filed Clarify documentation for current_root_span DataDog/dd-trace-py#9758 to see if there's even a known way to get ahold of the service entry span

(Note that this compounds with the trace concatenation issue -- attributes get attached to the initial trace that all the others get concatenated to.)

[timmc-edx]

Closing out -- I think we're actually OK, and the docs are just confusing. I've updated DataDog/dd-trace-py#9758 to be about clarifying the documentation. We can reopen if it turns out I was wrong.