When kafka rejects a message (perhaps the message is too big, or some other error) our monolog handler never knows because it uses the default ack=0, which basically just throws messages into the wind and hopes they arrive. We should update this to ack=1 which requires the partition leader to acknowledge the request.
The kafka library we use supports this, we simply need to set the value. We also need to look at what the error responses look like and log them on the mediawiki side. Kafka also logs these, but it's good to get the logging on both ends.