[messaging] clarify per message span attributes in case there is no parent information in the message

[Flarna]

The messaging span spec tells that for batch receive/process scenarios span attributes which differ per message should be set on the links describing the message.

How to do that in case the incoming message holds no producer span context? For example because there is no instrumentation in the producer side.

Should we add a span link with trace.id and span.id set to all 0?