@Override
  public OperationCallable<
          SynthesizeLongAudioRequest, SynthesizeLongAudioResponse, SynthesizeLongAudioMetadata>
      synthesizeLongAudioOperationCallable() {
    return synthesizeLongAudioOperationCallable;
  }
Constructs an instance of GrpcTextToSpeechLongAudioSynthesizeStub, 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/GrpcTextToSpeechLongAudioSynthesizeStub.java