Override BlockingPublisher Default Endpoint 

  namespace pubsub = ::google::cloud::pubsub;
  using ::google::cloud::Options;
  []() {
    // This service supports specifying a regional or locational endpoint prefix
    // when creating the SubscriptionAdminConnection.
    // For example, to connect to "europe-central2-pubsub.googleapis.com":
    auto pub = pubsub::BlockingPublisher(
        pubsub::MakeBlockingPublisherConnection("europe-central2"));

    // This configuration is common with Private Google Access:
    //     https://cloud.google.com/vpc/docs/private-google-access
    auto vpc_pub =
        pubsub::BlockingPublisher(pubsub::MakeBlockingPublisherConnection(
            Options{}.set<google::cloud::EndpointOption>(
                "private.googleapis.com")));
  }