Adding producer latency to Message Delivery Report

[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.

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[ivaaaan]

Are there any plans to merge this from Kafka team? We also faced this problem in our systems