Metrics are incomplete if error encountered parsing CSV from HAProxy

[beorn7]

This error could possibly even include a timeout, in which case we would prefer to not expose any metrics at all rather than an incomplete set (which can lead to heavy underreporting of traffic and thus cause false alerts). Essentially, in the scrape() method, we loop through the lines, setting metrics, and we will just skip errors there.

This will probably implicitly change/get fixed by moving to const metrics, see #43