@Override
  public void shutdown() {
    backgroundResources.shutdown();
  }
Constructs an instance of GrpcTpuStub, 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-tpu/google-cloud-tpu/src/main/java/com/google/cloud/tpu/v2/stub/GrpcTpuStub.java