[Docs] Describe which Kafka metadata is store in Broadway.Message

[José Victor Zeller]

Hello,

I was reading the docs looking through which metadata is added to Broadway.Message but couldn't find it anywhere besides the implementation (https://github.com/dashbitco/broadway_kafka/blob/v0.3.0/lib/broadway_kafka/producer.ex#L517).
It would be welcome if I open a PR adding it to the documentation?

[José Valim]

Yes please!