code: func NewBoundarySampler(rate float64, salt int64) (Sampler, error) {
  	if rate == 0.0 {
  		return NeverSample, nil
  	}
  	if rate == 1.0 {
  		return AlwaysSample, nil
  	}
  	if rate < 0.0001 || rate > 1 {
  		return nil, fmt.Errorf("rate should be 0.0 or between 0.0001 and 1: was %f", rate)
  	}
  
  	var (
  		boundary = int64(rate * 10000)
  		usalt    = uint64(salt)
  	)
  	return func(id uint64) bool {
  		return int64(math.Abs(float64(id^usalt)))%10000 < boundary
  	}, nil
  }
docs: // NewBoundarySampler is appropriate for high-traffic instrumentation who
  // provision random trace ids, and make the sampling decision only once.
  // It defends against nodes in the cluster selecting exactly the same ids.