fix: Set batching settings on SinglePartitionPublisher (#563)

Author: dpcollins-google

pubsublite-beam-io/src/main/java/com/google/cloud/pubsublite/beam/Publishers.java

@@ -26,6 +26,7 @@
 import com.google.cloud.pubsublite.AdminClientSettings;
 import com.google.cloud.pubsublite.MessageMetadata;
 import com.google.cloud.pubsublite.Partition;
+import com.google.cloud.pubsublite.cloudpubsub.PublisherSettings;
 import com.google.cloud.pubsublite.internal.Publisher;
 import com.google.cloud.pubsublite.internal.wire.PartitionCountWatchingPublisherSettings;
 import com.google.cloud.pubsublite.internal.wire.PubsubContext;
@@ -92,6 +93,7 @@ static Publisher<MessageMetadata> newPublisher(PublisherOptions options) throws
  .setTopic(options.topicPath())
  .setPartition(partition)
  .setServiceClient(newServiceClient(options, partition))
+ .setBatchingSettings(PublisherSettings.DEFAULT_BATCHING_SETTINGS)
  .build())
  .setAdminClient(newAdminClient(options))
  .build()