Add an interface declaration for Metrics

[bobbytables]

Also fixed some golint errors on the package level functions.

Purpose

Right now we use *metrics.Metrics as the type when we initialize things like servers or daemons with this package. This is fine in tests if you use a blackhole sink but it would be nice to just have something that guarantees the interface that we can implement ourselves for testing purposes. Since MockSink isn't exported. Alternatively we could just export the MockSink (its declared in a test file).

[bobbytables]

Ping, any thoughts on this?

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