protected final void blockingGetToCallback(URI uri, RequestMetadataCallback callback) {
    Map<String, List<String>> result;
    try {
+    Thread.sleep(5000);
      result = getRequestMetadata(uri);
    } catch (Throwable e) {
      callback.onFailure(e);
      return;
    }
    callback.onSuccess(result);
  }

$ ./run-test-server.sh

$ ./run-test-client.sh  --server_host_override=foo.test.google.fr --use_test_ca=true --test_case=oauth2_auth_token --service_account_key_file="/Users/zivy/.config/google-credentials/tensorflow-credential.json" --oauth_scope="https://www.googleapis.com/auth/cloud-platform"

Exception in thread "grpc-default-executor-0" java.util.concurrent.RejectedExecutionException: event executor terminated
	at io.netty.util.concurrent.SingleThreadEventExecutor.reject(SingleThreadEventExecutor.java:926)
	at io.netty.util.concurrent.SingleThreadEventExecutor.offerTask(SingleThreadEventExecutor.java:353)
	at io.netty.util.concurrent.SingleThreadEventExecutor.addTask(SingleThreadEventExecutor.java:346)
	at io.netty.util.concurrent.SingleThreadEventExecutor.execute(SingleThreadEventExecutor.java:828)
	at io.netty.util.concurrent.SingleThreadEventExecutor.execute(SingleThreadEventExecutor.java:818)
	at io.grpc.netty.WriteQueue.scheduleFlush(WriteQueue.java:67)
	at io.grpc.netty.WriteQueue.enqueue(WriteQueue.java:87)
	at io.grpc.netty.NettyClientStream$Sink.writeHeadersInternal(NettyClientStream.java:165)
	at io.grpc.netty.NettyClientStream$Sink.writeHeaders(NettyClientStream.java:121)
	at io.grpc.internal.AbstractClientStream.start(AbstractClientStream.java:159)
	at io.grpc.internal.DelayedStream.internalStart(DelayedStream.java:280)
	at io.grpc.internal.DelayedStream.setStream(DelayedStream.java:159)
	at io.grpc.internal.MetadataApplierImpl.finalizeWith(MetadataApplierImpl.java:104)
	at io.grpc.internal.MetadataApplierImpl.apply(MetadataApplierImpl.java:79)
	at io.grpc.CallCredentials2$1.apply(CallCredentials2.java:61)
	at io.grpc.auth.GoogleAuthLibraryCallCredentials$1.onSuccess(GoogleAuthLibraryCallCredentials.java:134)
	at com.google.auth.Credentials.blockingGetToCallback(Credentials.java:127)
	at com.google.auth.Credentials$1.run(Credentials.java:105)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)