docs: public static <T extends ContentMeta> ResourceSelector<T> exactMetadataResourceSelector(final Map<String,
              String> required, final boolean requireAll) {
          return new ResourceSelector<T>() {
              @Override
              public boolean matchesContent(T content) {
                  for (String key : required.keySet()) {
                      String expect = required.get(key);
                      String test = content.getMeta().get(key);
                      if (null != test && expect.equals(test)) {
                          if (!requireAll) {
                              return true;
                          }
                      } else if (requireAll) {
                          return false;
                      }
                  }
                  return requireAll;
              }
          };
      }