code: protected Objects.ToStringHelper toStringHelper() {
      return Objects.toStringHelper(this).add("messageNumber", messageNumber)
          .add("resourceState", resourceState).add("resourceId", resourceId)
          .add("resourceUri", resourceUri).add("channelId", channelId)
          .add("channelExpiration", channelExpiration).add("channelToken", channelToken)
          .add("changed", changed);
    }
docs: Returns the helper for {@link #toString()}.