Expose metrics to prometheus

As combination of prometheus + alertmnanager is widely taken for collecting metrics and monitoring cluster. It will be good expose some predefined metrics directly to prometheus. The benefit of doing it is making it easier to retrieve history metrics, improving data visibility and possibly uncover hidden relationship between different events. Besides prometheus is becoming a defacto tool for a k8s clsuter, enabling masakri to directly expose metrics to prometheus will help unify cloud monitoring tools and simply cloud operators work.