code: func InstrumentRoundTripperInFlight(gauge prometheus.Gauge, next http.RoundTripper) RoundTripperFunc {
  	return RoundTripperFunc(func(r *http.Request) (*http.Response, error) {
  		gauge.Inc()
  		defer gauge.Dec()
  		return next.RoundTrip(r)
  	})
  }
docs: // InstrumentRoundTripperInFlight is a middleware that wraps the provided
  // http.RoundTripper. It sets the provided prometheus.Gauge to the number of
  // requests currently handled by the wrapped http.RoundTripper.
  //
  // See the example for ExampleInstrumentRoundTripperDuration for example usage.