google-cloud-java: PubSub runs into `java.lang.RuntimeException: ManagedChannel allocation site` when a new publisher is created

Hi!

We’ve currently received around 100-200 PubSub related issues when a PubSub publisher is created with the following stacktrace:

io.grpc.internal.ManagedChannelOrphanWrapper$ManagedChannelReference cleanQueue: *~*~*~ Channel ManagedChannelImpl{logId=4613, target=pubsub.googleapis.com:443} was not shutdown properly!!! ~*~*~* (ManagedChannelOrphanWrapper.java:163)
    Make sure to call shutdown()/shutdownNow() and wait until awaitTermination() returns true.
java.lang.RuntimeException: ManagedChannel allocation site
	at io.grpc.internal.ManagedChannelOrphanWrapper$ManagedChannelReference.<init>(ManagedChannelOrphanWrapper.java:103)
	at io.grpc.internal.ManagedChannelOrphanWrapper.<init>(ManagedChannelOrphanWrapper.java:53)
	at io.grpc.internal.ManagedChannelOrphanWrapper.<init>(ManagedChannelOrphanWrapper.java:44)
	at io.grpc.internal.AbstractManagedChannelImplBuilder.build(AbstractManagedChannelImplBuilder.java:410)
	at com.google.api.gax.grpc.InstantiatingGrpcChannelProvider.createSingleChannel(InstantiatingGrpcChannelProvider.java:206)
	at com.google.api.gax.grpc.InstantiatingGrpcChannelProvider.createChannel(InstantiatingGrpcChannelProvider.java:162)
	at com.google.api.gax.grpc.InstantiatingGrpcChannelProvider.getTransportChannel(InstantiatingGrpcChannelProvider.java:149)
	at com.google.api.gax.rpc.ClientContext.create(ClientContext.java:151)
	at com.google.cloud.pubsub.v1.stub.GrpcPublisherStub.create(GrpcPublisherStub.java:161)
	at com.google.cloud.pubsub.v1.Publisher.<init>(Publisher.java:154)
	at com.google.cloud.pubsub.v1.Publisher.<init>(Publisher.java:83)
	at com.google.cloud.pubsub.v1.Publisher$Builder.build(Publisher.java:607)

We are using PubSub library v 1.40.0. And here is how we create publishers:

Publisher createPublisher(ProjectTopicName topicName) {
        try {
            Publisher publisher = Publisher.newBuilder(topicName)
                                           .build();
            return publisher;
        } catch (IOException e) {
            String errorMessage = format("Cannot create a publisher for topic %s", topicName);
            throw new IllegalStateException(errorMessage, e);
        }
    }

Any thoughts or suggestions? By the way, we are running on AppEngine Standard with Java 8.