storage/buffer.go
func (r *sampleRing) iterator() SeriesIterator {
	r.it.r = r
	r.it.i = -1
	return &r.it
}
func (r *sampleRing) iterator() SeriesIterator {
	r.it.r = r
	r.it.i = -1
	return &r.it
}
// Returns the current iterator. Invalidates previously returned iterators.