I just installed XCP-ng on a new server and wanted Prometheus metrics too, so I wrote this: https://github.com/MikeDombo/xen-exporter.

It is a simple Python script (which can run in Docker) that collects the metrics from XCP-ng's RRD API. These are the same metrics that you'd see in Xen Orchestra or in the XenServer Console application. In addition to the exporter, I've also created a Grafana dashboard to visualize these metrics too.