code: func ValidKeyToken(s string) bool {
  	if !utf8.ValidString(s) {
  		return false
  	}
  	for _, r := range s {
  		if !unicode.IsPrint(r) || r == unicode.ReplacementChar {
  			return false
  		}
  	}
  	return true
  }
docs: // ValidKeyToken returns true if the token used for measurement, tag key, or tag
  // value is a valid unicode string and only contains printable, non-replacement characters.