docs: public <T extends Enum<T>> Optional<ImmutableSet<T>> getEnumSet(String key, Class<T> clazz) {
      return this.get(key)
          .map(
              value ->
                  Streams.stream(Splitter.on(',').omitEmptyStrings().split(value))
                      .map(v -> asEnumValue(key, v, clazz))
                      .collect(toImmutableSet()));
    }