Change metrics abstraction to be more performant

[ejsmith]

Our current metrics abstraction forces users to pass the name of the metric with every single call to update the metric (increment, decrement, etc). We should be able to define the configuration for our metrics as a static or singleton somewhere and then just call methods on it like Increment() without having to have any string allocations.

Also, we need to support adding labels / dimensions to metrics for things like hostname, etc. Possibly support metric scopes for defining labels that should be added to all metrics within it (#114 #32).