fix: add header providers to PublisherStubSettings (#294)

Author: vinnyspb

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

@@ -171,7 +171,8 @@ private Publisher(Builder builder) throws IOException {
             .setCredentialsProvider(builder.credentialsProvider)
             .setExecutorProvider(FixedExecutorProvider.create(executor))
             .setTransportChannelProvider(builder.channelProvider)
-            .setEndpoint(builder.endpoint);
+            .setEndpoint(builder.endpoint)
+            .setHeaderProvider(builder.headerProvider);
     stubSettings
         .publishSettings()
         .setRetryableCodes(