More robust HAProxy collector with simpler config

This feature does two main things:
1. It uses a proper CSV-parsing library to parse the output from HAProxy, which makes it more robust in case there are ever differences in the output format from different versions of HAProxy (since it does not rely on indices on a line staying the same, as the current one does).
2. It makes it easier to configure the collector for reporting additional values, by simply adding the name of the metric to a list defined at the top of the code.