Messaging: prefetch scenario observability

[lmolkova]

Related to #163 (comment)

Some messaging libraries and systems support push-model and could have prefetch queue:

• client library receives a message but does not always dispatch it to the application code right away
• message can stay in the prefetch queue for some time and then could be dispatched to the application

We get a lot of question\problems reported by users on it in Azure Messaging ServiceBus SDK and would find it useful to know if the message ever reached the consumer and know how much time it spent in the queue waiting to be dispatched to the application code.

Having some traces/events and metrics on it would help identify configuration problems, tune client library settings and debug disappearing messages.