[CD18] Document Prometheus jmx-exporter use with EAP images

The EAP images in the 6.4.x, 7.1.x, 7.2.x and CD streams now include support for providing Prometheus[1] format metrics via the Prometheus jmx-exporter java agent.[2]

The images include:

• The java binaries for the jmx-exporter java agent
• A configuration yaml file with configuration for the agent appropriate for the given image (i.e. what metrics to expose and how those map to the JMX mbeans available in that release)
• Scripting that will configure use of the agent in the EAP jvm launch that is executed when the container starts.

User control of the use of the agent is accomplished using 3 environment variables:

• AB_PROMETHEUS_ENABLE – set to 'true' to turn on use of the agent. Default is 'false'
• AB_PROMETHEUS_JMX_EXPORTER_PORT – port on which the jmx-exporter agent should listen for scrapes from a Prometheus server. Default is 9799. The agent listens on localhost so if the user wants the metrics to be available outside the container their application's DeploymentConfig will need to include Service exposing this endpoint.
• AB_PROMETHEUS_JMX_EXPORTER_CONFIG – path within the container to a configuration.yaml file to use instead of the one we provide. I don't expect this to be a common scenario. The s2i mechanism for getting content like this into the image is documented at [3]. For example, if a source directory listed in the CUSTOM_INSTALL_DIRECTORIES env var had a 'configuration' dir with a 'my-prometheus-config.yaml' file, that file would end up in /opt/eap/standalone/configuration/my-prometheus-config.yaml'.

[1] https://prometheus.io/docs/introduction/overview/
[2] https://github.com/prometheus/jmx_exporter
[3] https://access.redhat.com/documentation/en-us/jboss_enterprise_application_platform_continuous_delivery/15/html-single/getting_started_with_jboss_eap_for_openshift_container_platform/#S2I-Artifacts