DeliveryReport attached to Message for Producer Calls

[AdrielVelazquez]

Originally in response to this message, and better metrics internally.

https://github.com/confluentinc/confluent-kafka-go/issues/425#issue-559017454

Background:

Our internal applications measure producer latency, by consuming directly from the topic and producing a diff of the message timestamp, and the event timestamp. Exposing the Producer Latency directly to the delivery report will help prevent having an unnecessary consumer to report on Producer Latency.

Revamped version of this

https://github.com/confluentinc/confluent-kafka-go/pull/1076