Memory leak in PrometheusKieServerExtension

Prometheus Kie server extensions contains support for custom metrics [1].
These metrics are added when a new container is called [2].
However when container is removed then these metrics stay active. Agenda event listener has container instance as a HashMap key. This container instance is quite big, causing memory leak.

[1] https://github.com/kiegroup/droolsjbpm-integration/blob/master/kie-server-parent/kie-server-services/kie-server-services-prometheus/src/main/java/org/kie/server/services/prometheus/PrometheusCustomMetricsSupport.java
[2] https://github.com/kiegroup/droolsjbpm-integration/blob/master/kie-server-parent/kie-server-services/kie-server-services-prometheus/src/main/java/org/kie/server/services/prometheus/PrometheusCustomMetricsSupport.java#L78-L94