query/store: memoize PromLabels() call

[GiedriusS]

We use the stringlabels call so some allocations are inevitable but we can be much smarter about it:

func (s *storeSeriesSet) At() (labels.Labels, []*storepb.AggrChunk) {
	return s.series[s.i].PromLabels(), s.series[s.i].Chunks <--- not memoized, new alloc on every At() call; need to memoize because of stringlabel. One alloc is inevitable.
}

lset, chks := s.SeriesSet.At()
if s.peek == nil {
	s.peek = &Series{Labels: labelpb.PromLabelsToLabelpbLabels(lset), Chunks: chks} <-- converting back to labelpb ?
	continue
}

if labels.Compare(lset, s.peek.PromLabels()) != 0 { <--- PromLabels() called; we can avoid this call
	s.lset, s.chunks = s.peek.PromLabels(), s.peek.Chunks <- PromLabels() called; we can avoid this
	s.peek = &Series{Labels: labelpb.PromLabelsToLabelpbLabels(lset), Chunks: chks} <--- converting back to labelpb; we can avoid this
	return true
}

[pedro-stanaka]

lg