code: func NewExpDecaySample(reservoirSize int, alpha float64) Sample {
  	if !Enabled {
  		return NilSample{}
  	}
  	s := &ExpDecaySample{
  		alpha:         alpha,
  		reservoirSize: reservoirSize,
  		t0:            time.Now(),
  		values:        newExpDecaySampleHeap(reservoirSize),
  	}
  	s.t1 = s.t0.Add(rescaleThreshold)
  	return s
  }
docs: // NewExpDecaySample constructs a new exponentially-decaying sample with the
  // given reservoir size and alpha.