code: func (l *Listener) trackConn() func() {
  	c := atomic.AddInt32(&l.activeConns, 1)
  	metrics.SetGaugeWithLabels([]string{l.metricPrefix, "conns"}, float32(c),
  		l.metricLabels)
  
  	return func() {
  		c := atomic.AddInt32(&l.activeConns, -1)
  		metrics.SetGaugeWithLabels([]string{l.metricPrefix, "conns"}, float32(c),
  			l.metricLabels)
  	}
  }
docs: // trackConn increments the count of active conns and returns a func() that can
  // be deferred on to decrement the counter again on connection close.