type ActiveWorkerStat struct {
	Curr int     `json:"curr"`
	Avg  float32 `json:"avg"`
	Max  int     `json:"max"`
	hist metrics.Histogram
}

func newActiveWorkerStat(r metrics.Registry) *ActiveWorkerStat {
	h := metrics.NewHistogram(metrics.NewUniformSample(100))
	r.Register("replication.active_workers", h)
	return &ActiveWorkerStat{
		hist: h,
	}
}

// update curr and max workers;