RabbitMQ driver enhancements

[itrofimow]

Would be great to see the following features implemented in RabbitMQ driver:

1. Dynamic config with some sane timeout defaults
2. Headers in both publish and admin methods (headers for publish methods might be convenient for additional metadata, headers for admin methods allow one to do this , for example)
3. Metrics - as of now they are mostly just a bunch of placeholders, and they could be way more useful

Continuation of https://github.com/userver-framework/userver/issues/44, basically