metrics.go
func ServeMetrics(ctx context.Context, l net.Listener) error {
	return http.Serve(l, promhttp.Handler())
}
func ServeMetrics(ctx context.Context, l net.Listener) error {
	return http.Serve(l, promhttp.Handler())
}
// ServeMetrics serves Prometheus metrics endpoint on the
// provided net.Listener
// Use for long-running services.  For cli tools, use PushMetrics instead.