Document the Kie Server Prometheus Extension

Related to:

https://issues.jboss.org/browse/BAPL-881

Write the technical documentation related to the integration between the kie-server and Prometheus.

This should cover:

• Why the user should do that, and what's Prometheus for

Prometheus is a time based database used to store metrics related to the execution of DMN models or Drools rules that can be graphed using various tool such as Grafana

• How to enable the prometheus extension in both kie.server and spring-boot (they're a bit different)

To enable the prometheus extension in the kie-server one should

Add the required kie server extension to the maven's pom.xml

<dependency>
      <groupId>org.kie.server</groupId>
      <artifactId>kie-server-services-prometheus</artifactId>
    </dependency>

    <dependency>
      <groupId>org.kie.server</groupId>
      <artifactId>kie-server-rest-prometheus</artifactId>
    </dependency>
    <dependency>

set the environment variable ```org.kie.prometheus.server.ext.disabled``` to false

on Spring boot, configure this key on application.properties

kieserver.drools.enabled=true
kieserver.dmn.enabled=true
kieserver.prometheus.enabled=true

You'll need both drools and Prometheus (or DMN and Prometheus) to have the extension running.

• How to configure prometheus to scrape metrics from the kie-server

Configure the prometheus.yaml accordingly, i.e.

scrape_configs:     metrics_path: /rest/metrics
    static_configs:       - targets: ['localhost:8090']

The endpoint will always be `/rest/metrics` while the target will depend on the application server in which the kie server was deployed.

• How to set up an Openshift image

Use the Openshift image with the Kie-server