instruments.go
func (r *Reservoir) Snapshot() Distribution {
	h := newHistogram(defaultHistogramSize)
	r.m.Lock()
	h = r.hist.Copy(h)
	r.m.Unlock()
	return h
}
func (r *Reservoir) Snapshot() Distribution {
	h := newHistogram(defaultHistogramSize)
	r.m.Lock()
	h = r.hist.Copy(h)
	r.m.Unlock()
	return h
}
// Snapshot returns a Distribution