fix(haproxy): don't use lsof, avoid dashes and dots in metric names

This changes the haproxy plugin so that:
- it does not rely on lsof to count tcp connections (they would be counted twice and would slow the plugin down a lot) and uses the stats socket instead
- adds a metric to report the global rate of connections
- changes the sample alarm code so that it actually works (metric names are case sensitive)
- makes sure per-backend metrics never contain dashes nor dots as they confuse the agent