Override @c Publisher Default Endpoint

 namespace pubsub = ::google::cloud::pubsub;
  using ::google::cloud::Options;
  [](std::string const& project_id, std::string const& topic_id) {
    // 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::Publisher(pubsub::MakePublisherConnection(
        "europe-central2", pubsub::Topic(project_id, topic_id)));

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