Callback-accepting Sink/Observable?

[clayrat]

@alexandru
I feel it's a somewhat common scenario to want to do something after writing to Kafka - like send some sort of metrics for uploaded data, or acknowledge the write in some kind of a database. However, it seems this is not really possible with the existing facilities. Do you think we could write some kind of callback-accepting version of KafkaProducerSinks or even make an Observable version of it?