Support producer name for Message

[wolfstudy]

Signed-off-by: xiaolong.ran rxl@apache.org

Fixes #295

Motivation

Support producer name for Message interface, allows the user to get the producerName of the message in the received message.

msg, err := consumer.Receive(context.Background())
if err != nil {
	log.Fatal(err)
}
	
log.Println(msg.ProducerName())

Modifications

• add producerName filed to message struct

[wolfstudy]

@merlimat Hello, currently, the Jenkinns CI still running, Do we need to do anything else for this?

[wolfstudy]

@JoseFMP PTAL thanks

[merlimat]

Saying "from broker" can be misleading since we're not fetching it from the broker, rather the name comes attached to the message metadata.

In Java client doc we have:

Get the producer name who produced this message.

Suggested change

	// ProducerName will get producer name from broker
	// ProducerName returns the name of the producer that has published the message