promql/lex.go
func isLabel(s string) bool {
	if len(s) == 0 || !isAlpha(rune(s[0])) {
		return false
	}
	for _, c := range s[1:] {
		if !isAlphaNumeric(c) {
			return false
		}
	}
	return true
}
func isLabel(s string) bool {
	if len(s) == 0 || !isAlpha(rune(s[0])) {
		return false
	}
	for _, c := range s[1:] {
		if !isAlphaNumeric(c) {
			return false
		}
	}
	return true
}
// isLabel reports whether the string can be used as label.