Metrics and logging

This is mostly a cosmetic PR, it changes the logger to use `logrus` exported funcs. It also uses `expvar` for metrics, which the [upcoming Datadog agent (v5.0)](https://github.com/DataDog/dd-agent/blob/73dd75035f1982d31818bb8a4a2a6cc856cf8bb7/CHANGELOG.md#500--unreleased) will plug into out of the box.

@fbogsany 
The logic changes to `Fatal` here because I've noticed that a file with improper data will not start to magically contain proper data. Even if it would, the better thing would be to abort and inspect the file manually.