code: func (a Dimensions) Normalize() (time.Duration, []string) {
  	var dur time.Duration
  	var tags []string
  
  	for _, dim := range a {
  		switch expr := dim.Expr.(type) {
  		case *Call:
  			lit, _ := expr.Args[0].(*DurationLiteral)
  			dur = lit.Val
  		case *VarRef:
  			tags = append(tags, expr.Val)
  		}
  	}
  
  	return dur, tags
  }
docs: // Normalize returns the interval and tag dimensions separately.
  // Returns 0 if no time interval is specified.