Expose rules.NewGroupMetrics and rules.Metrics. (#5059)
Expose rules.NewGroupMetrics and rules.Metrics. (#5059)

Signed-off-by: Tom Wilkie <4d61d701e7f49021437d91fbf6a1826e6d4e51de@gmail.com>
Gouthamve/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,alileza/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,Gouthamve/prometheus,kangwoo/prometheus,keegancsmith/prometheus,prometheus/prometheus,alileza/prometheus,keegancsmith/prometheus,prometheus/prometheus,keegancsmith/prometheus,prometheus/prometheus,kangwoo/prometheus,Gouthamve/prometheus,kangwoo/prometheus,kangwoo/prometheus,Gouthamve/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus