code: private String[] nullToEmpty(String... labelValues) {
          return Arrays.stream(labelValues)
                  .map(labelValue -> null == labelValue ? "" : labelValue)
                  .toArray(String[]::new);
      }
docs: Prometheus doesn't permit {@code null} labels, so replace them with empty strings.
  @param labelValues the label values, which may contain {@code null} elements
  @return the label values with {@code null} elements replaced with empty strings