A Builder for MetricSets. Create a MetricSet by calling the metricset
method, passing in a Closure defining the contents of the MetricSet.
The Closure can contain any combination of the following (as well as
arbitrary Groovy code):
metricset - to load a MetricSet file. The path specifies a Groovy file.metric - to load a single Metric; specify the Metric classdescription - description of the MetricSet (optional)