code: public static Sample ofIsoText(String isoDate, String textValue) {
          return new Sample(null, isoDate, null, textValue);
      }
docs: Creates a new {@link Sample} with date in ISO 8061 format and text value specified
  
  @param  isoDate      date in ISO 8061 format according
  to <a href="https://www.ietf.org/rfc/rfc3339.txt">RFC3339</a>
  @param  textValue    the text value of the sample
  @return the Sample with specified fields