:message_count is not honoured as metadata option.

[cevado]

Current documentation shows :message_count as an available option for metadata of a message. I belive this is not possible to achieve with current implementation. message_count is only available as a metadata when doing basic get(not recommended by the way), current implementation uses basic consume that doesn't expose message_count as a metadata.
Possible see solutions:

• just remove it from the docs.
• use AMQP.Queue.message_count/2 to populate the message_count metadata.

I can do a fast PR for fixing it. just need to know what is the preferred approach.

[José Valim]

Let’s update the docs to remove it for now and we add it back if someone ever needs it. :)