Add kafka.producer. and kafka.consumer Datadog metrics

When you enable the Datadog integration in Aiven for Apache Kafka®, the service supports all of the broker-side metrics listed in the Datadog Kafka integration documentation and allows you to send additional custom metrics.

However, all metrics that have a prefix like kafka.producer.* or kafka.consumer.* are client-side metrics that should be collected from the producer or consumer, and sent to Datadog.

The dedicated Datadog documentation (see “Missing producer and consumer metrics” chapter) provides a way to include the missing metrics natively for Java based producers and consumers or via DogStatsD for clients in other languages.