code: public static <T> MetadataKey<T> repeated(String label, Class<T> clazz) {
      return new MetadataKey<T>(label, clazz, true);
    }
docs: Creates a key for a repeated piece of metadata. If metadata is added more than once using
  this key for a log statement, all values will be retained as key/value pairs in the order
  they were added.
  <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.