Metrics and logging

WIP: do not review

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.