Propagate context to Spring MessageListenerAdapter

[richardstartin]

When Spring users use RabbitMQ MessageListenerAdapter they get broken traces, because the scope of the basic.deliver span is not propagated to the MessageListenerAdapter's delegate, where any traceable activity will start a new trace. This happens because Spring AMQP (wisely) invokes user callbacks on a different thread to the MQ subscription, and the handoff is not done via an executor. Instead, RabbitMQ Envelopes are wrapped in Delivery objects which are enqueued by BlockingQueueConsumer, which drains said queue and wraps the Deliverys as Message objects, which will eventually be passed to the MessageListenerAdapter via AbstractMessageListenerContainer.executeListener.

The basic data flow here

• capture any active scope and field inject it into Delivery in construction (in the subscription thread)
• transfer state from Delivery to Message in BlockingQueueConsumer.handle (on the handling thread)
• extract and activate the continuation in AbstractMessageListenerContainer.executeListener and close the scope afterwards.

Since this instrumentation depends on two libraries (spring-rabbit and spring-amqp) it is muzzled via spring-boot-starter-amqp which depends on both.

Since there is a circular dependency between this instrumentation and rabbit-amqp-2.7, it needs to be tested that this instrumentation does not interfere with the other instrumentation, but this cannot be done by adding a dependency both ways. I copied the rabbitmq tests into this module and they pass here, but removed the copied test before committing. I can add it back if it's felt important enough.