[FEATURE REQ] Messaging tracing: support disabling per-message tracing

[lmolkova]

Library name

Azure.Messaging.EventHubs, Azure.Messaging.ServiceBus

Please describe the feature.

We had a several questions/issues from users who didn't like message+send spans. In case there is just one message in a batch, then having two spans does not bring any value.
Also, when batch size it huge, per-message tracing becomes too hard and might not make sense.

OTel semconv allow to create a span per message in a batch and a publish span OR create one publish span and reuse publish context on each message being published.

We can do the following:

1. if user enabled per-message source, then we'll create span per message
   • if sender source is enabled, we'll also create a publish span - this existing path, no changes are expected
2. if user didn't enable per-message source, we won't create a span per message
   • if sender source is enabled, we'll create a publish span:
     • if some message have pre-existing context provided by user, we won't update it and will link to it
     • otherwise, we'll inject publish span context into the message
     • this is the new feature

• if sender source is not enabled - no tracing or context propagation happens - same as before.

[github-actions]

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