Prometheus is really nice. It focuses on the right things needed to monitor distributed systems (complex expressions and alerting) where others (e.g. opentsdb and influxdb) fall short, at the expense of deferring the issue of massive scalability which, let's face it, it's not a real requirement for most scenarios.

This looks really interesting - thanks for sharing!

Thats pretty cool, can it be used with HA proxy?

Yes, I've written up a blog post on how we do it: http://www.boxever.com/haproxy-monitoring-with-prometheus

It's been very useful.