code: public static <T> MetadataKey<T> single(String label, Class<T> clazz) {
      return new MetadataKey<T>(label, clazz, false);
    }
docs: Creates a key for a single piece of metadata. If metadata is set more than once using this
  key for the same log statement, the last set value will be the one used, and other values
  will be ignored (although callers should never rely on this behavior).
  <p>
  Key instances behave like singletons, and two key instances with the same label will still
  be considered distinct. The recommended approach is to always assign {@code MetadataKey}
  instances to static final constants.