We added support for monitoring the prometheus varnish exporter as part of T283660, and it is working properly. For example, I have seen the following resolution message on IRC:
< jinxer-wm> (VarnishPrometheusExporterDown) resolved: Varnish Exporter on instance cp2027:9331 is unreachable - https://alerts.wikimedia.org
The alert currently lacks a runbook (ie: what to do when the alert fires?) as well as a link to the relevant Grafana dashboard.