opentracing enhancement: add AMQP implementation to tracing middleware

[atkachyshyn]

In the current opentracing package there is a lack of the middleware for Publisher\Consumer span kind and logic for injection\extraction of the span context into\from the message. In the project, I'm working on I had to write my own implementation, but I believe it could be included in the go-kit's opentracing package if somebody else needs it.
I can create such a pull request if it fits into your roadmap.