Add Kafka producer and flush() prom metrics

[sotojn]

We should add metrics that are returned in the producer clients stats event and pass some of them into prom metrics. It may also be useful to add a metic that counts the total amount of times the Kafka producer flush() function is called to see if it's excessively called. This may be important because every flush call blocks the processor from writing any messages to the queue until the flush callback completes.

https://github.com/Blizzard/node-rdkafka/blob/master/README.md