code: func NewCounterFunc(opts prometheus.CounterOpts, function func() float64) prometheus.CounterFunc {
  	g := prometheus.NewCounterFunc(opts, function)
  	prometheus.MustRegister(g)
  	return g
  }
docs: // NewCounterFunc works like the function of the same name in the prometheus
  // package but it automatically registers the CounterFunc with the
  // prometheus.DefaultRegisterer. If the registration fails, NewCounterFunc
  // panics.