Add more detailed document on how the exporters work

[busma13]

@godber suggested here that we need better exporter docs. I will take this on, but I will need to learn a lot more about all the exporters first.

[busma13]

Jeff's descriptions to be merged with what's in the readme already:

kafka-jmx-agent-exporter

• Runs in the kafka broker container attached to the java process.
• This is the method we use on our kafka cluster.
• Each broker has a kafka-jmx-agent-exporter.
• Port 5557
• Link https://github.com/prometheus/jmx_exporter

Metrics

jvm_*
kafka_network_*
kafka_controller_*
kafka_server_*
process_*

kafka-jmx-exporter

• Runs in a container of the broker pod
• ⚠️ We have not used this exporter, and the version referenced in the chart is very old. There most likely a more recent and maintained version.
• Port 5557
• Link https://hub.docker.com/r/solsson/kafka-prometheus-jmx-exporter

Metrics

kafka_network_*
kafka_controller_*
kafka_server_*
process_*

kafka-exporter

• Deployed as a separated pod in the same name space as the kafka cluster.
• There is only one kafka-exporter deployed per cluster.
• Port 9308

Link https://github.com/danielqsj/kafka_exporter

Metrics

go_*
kafka_broker_info
kafka_brokers
kafka_consumergroup_*
kafka_topic_partition_*
process_*
promhttp_*