www/varnish_exporter: create port

Varnish exporter for Prometheus. Scrapes the varnishstat -j JSON output on each
Prometheus collect and exposes all reported metrics.

WWW: https://github.com/jonnenauha/prometheus_varnish_exporter

PR: 230177
Submitted by: Žygis Škulteckis <ports@zx23.net> (maintainer)