@Override
  public OperationCallable<
          SynthesizeLongAudioRequest, SynthesizeLongAudioResponse, SynthesizeLongAudioMetadata>
      synthesizeLongAudioOperationCallable() {
    return synthesizeLongAudioOperationCallable;
  }
Constructs an instance of HttpJsonTextToSpeechLongAudioSynthesizeStub, using the given
settings. This is protected so that it is easy to make a subclass, but otherwise, the static
factory methods should be preferred.
java-texttospeech/google-cloud-texttospeech/src/main/java/com/google/cloud/texttospeech/v1/stub/HttpJsonTextToSpeechLongAudioSynthesizeStub.java