Upgrade armon/go-metrics to hashicorp/go-metrics

[Dentrax]

Fixes #707

[hashicorp-cla-app]

All committers have signed the CLA.

[rboyer]

Quoting @jmurret's reply on another nearly identical PR:

Hi all, we will post a more detailed thread on why this is complicated, but the short gist is that this change has to be consistently updated across the dependency chains of libraries used in an application. go-metrics relies on a singleton sink in global state. If any applications have a mix of armon metrics and hashicorp due to underlying dependencies not aligning on only one of them, metrics will be split among different sinks in the global state.
This change has to be coordinated with changing and releasing this dependency across multiple libraries and applications in a thoughtful way. This PR is taking longer than expected due to the prioritization and coordination of those efforts to ensure a smooth transition. Thank you for your patience and understanding.