v6.0.0-alpha.1

This is an alpha or prerelease or something like that. It includes a fix for config files (#52) and a major overhaul of the way we parse fields from rt.fastly.com and map them to Prometheus metrics (#51). As part of this overhaul we include a lot more new fields, and also change the name of some metrics to be more Prometheus-standards-compliant. As a result this is a breaking change and that's why this is an alpha release. If current users could give it a try and report any major issues I would appreciate it.