* Add metrics package with minimal copy of github.com/prometheus/common/model
Package metrics is a minimal copy of github.com/prometheus/common/model
providing types to work with the Prometheus-style results in a DigitalOcean
Monitoring metrics response.
We have copied this here as Prometheus' common packages are considered
internal to Prometheus, without any stability guarantees for external usage.
* Add support for retrieving Droplet monitoring metrics.
* Use pointer receivers.
Andrew Starr-Bochicchio