Metrics: Add kafka delivery error metric

Created by: danesss

We currently report metrics on "data loss errors" by detecting if the producer buffer is full.

This PR adds an additional metric kafka_delivery_errors. These errors are produced when the Kafka producer permanently fails delivery of a message. Delivery errors may eventually be captured by the existing "data loss errors" metric, but the new metric is a more direct way of capturing them, and can probably capture other errors ignored until now.