metrics_handler.go
func NewMetricsHandler(registry prometheus.Registerer, namespace string) Handler {
	return &metricsHandler{
		handler:   NewHandler(),
		registry:  registry,
		namespace: namespace,
	}
}
func NewMetricsHandler(registry prometheus.Registerer, namespace string) Handler {
	return &metricsHandler{
		handler:   NewHandler(),
		registry:  registry,
		namespace: namespace,
	}
}
// NewMetricsHandler returns a healthcheck Handler that also exposes metrics
// into the provided Prometheus registry.