feat: add randomly generated UUID to outgoing initial streaming pull requests (#77)

* google-cloud-pubsub/src/test/java/com/google/cloud/pubsub/v1/FakePublisherServiceImpl.java

* Ensure that if a batch is started and the timeout completes before the currently outstanding message has finished publishing with an ordering key that the last batch does in fact get published.

* add back in unit test

* feat: add randomly generated UUID to outgoing initial streaming pull requests for better ordering keys affinity

Author: kamalaboulhosn

google-cloud-pubsub/src/main/java/com/google/cloud/pubsub/v1/StreamingSubscriberConnection.java

@@ -45,6 +45,7 @@
 import com.google.pubsub.v1.StreamingPullResponse;
 import io.grpc.Status;
 import java.util.List;
+import java.util.UUID;
 import java.util.concurrent.ScheduledExecutorService;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicLong;
@@ -77,6 +78,13 @@ final class StreamingSubscriberConnection extends AbstractApiService implements
   private final Lock lock = new ReentrantLock();
   private ClientStream<StreamingPullRequest> clientStream;
 
+  /**
+   * The same clientId is used across all streaming pull connections that are created. This is
+   * intentional, as it indicates to the server that any guarantees made for a stream that
+   * disconnected will be made for the stream that is created to replace it.
+   */
+  private final String clientId = UUID.randomUUID().toString();
+
   public StreamingSubscriberConnection(
       String subscription,
       MessageReceiver receiver,
@@ -200,6 +208,7 @@ private void initialize() {
         StreamingPullRequest.newBuilder()
             .setSubscription(subscription)
             .setStreamAckDeadlineSeconds(60)
+            .setClientId(clientId)
             .build());
 
     /**