docs(metrics): updates for kafka exporter

[PaulRMellor]

Documentation

• Creates a separate section for the Kafka Exporter to bring together consumer lag concepts and a new procedure to enable the Kafka Exporter
• Removes the Kafka Exporter config step from the procedure for enabling the Prometheus JMX Exporter so that the Kafka Exporter option is not embedded within that section and treated as a separate option (whether JMX Exporter or Metrics Reporter is being used)

Checklist

Please go through this checklist and make sure all applicable tasks have been done

• Write tests
• Make sure all tests pass
• Update documentation
• Check RBAC rights for Kubernetes / OpenShift roles
• Try your changes from Pod inside your Kubernetes and OpenShift cluster, not just locally
• Reference relevant issue(s) and close them after merging
• Update CHANGELOG.md
• Supply screenshots for visual changes, such as Grafana dashboards

[fvaleri]

Thanks @PaulRMellor. This avoids any confusion between Prometheus JMX Exporter and Kafka Exporter.

[ppatierno]

Suggested change

	Kafka Exporter relies on data from the `__consumer_offsets` topic to report lag metrics.
	Kafka Exporter relies on data from the `__consumer_offsets` topic to report consumer lag metrics.

[ppatierno]

Suggested change

	Kafka Exporter relies on data from the `__consumer_offsets` topic to report lag metrics.
	Kafka Exporter relies on data from the `__consumer_offsets` topic to report consumer lag metrics.

[ppatierno]

@PaulRMellor can you rebase this PR please? There was a fix in main to allow the build to pass. Thanks!

[OwenCorrigan76]

From what I can see, this looks good.