code: func (ev *evaluator) evalSubquery(subq *SubqueryExpr) *MatrixSelector {
  	val := ev.eval(subq).(Matrix)
  	ms := &MatrixSelector{
  		Range:  subq.Range,
  		Offset: subq.Offset,
  		series: make([]storage.Series, 0, len(val)),
  	}
  	for _, s := range val {
  		ms.series = append(ms.series, NewStorageSeries(s))
  	}
  	return ms
  }
docs: // evalSubquery evaluates given SubqueryExpr and returns an equivalent
  // evaluated MatrixSelector in its place. Note that the Name and LabelMatchers are not set.