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 
I've actually deleted this in a later branch, because the flags get to contain keys and tokens, which I don't want printed in the logs. So I'll leave it in this change list; it's already fixed.