Kafka network throttling-related issues are hard to detect

[k32]

Kafka has a setting that introduces rate limiting for clients. kafka_protocol could be more informative in the situations when this happens

[zmstone]

Which version?
As far as I know, kafka dose this by sending back a non zero in throttle_time_ms field.
If we are to throttle connection process, it’s going to be unfair because

1. brod sends per partition produce requests, the protocol allows collective message sets for different topic-partitions. presumably it is unfair for brod if throttling is rate based.
2. Connection is shared between different topic-partitions in brod.

Maybe this is something for brod to fix, but not here in kafka_protocol.