Add MetricBuilder Static Types

[JHWelch]

To make this a little easier to work with with static analysis, looked to improve the typing of MetricBuilder. This adds the @extend annotation so that the underlying Builder instance will indicate it is outputting the Metric.

Using the @template tag allows for MetricBuilder to be overridden in the consuming app to reference an overridden Metric.

While looking at times handled a few other small items.

Thanks for the package! Let me know if you'd like me to handle anything differently.