Doc: Note limitations around Red Hat support of Prometheus and Grafana

Although we provide a metrics plugin for AMQ Broker that allows users to configure Prometheus metrics collection, we do not provide support for Prometheus. In addition, although we tell users that they can configure Grafana dashboards for metrics visualization, we don't provide direct support for Grafana.

This needs to be noted in the doc.

https://issues.redhat.com/browse/ENTMQBR-3599 has been opened to request that Broker provide example configurations for Prometheus and Grafana, similar to what Streams and Interconnect provides. These configuration examples need to be documented, when available.