code: func InstrumentHandlerInFlight(g prometheus.Gauge, next http.Handler) http.Handler {
  	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
  		g.Inc()
  		defer g.Dec()
  		next.ServeHTTP(w, r)
  	})
  }
docs: // InstrumentHandlerInFlight is a middleware that wraps the provided
  // http.Handler. It sets the provided prometheus.Gauge to the number of
  // requests currently handled by the wrapped http.Handler.
  //
  // See the example for InstrumentHandlerDuration for example usage.