code: public PubsubFuture<SubscriptionList> listSubscriptions(final String project,
                                                            final String pageToken) {
      final String query = (pageToken == null) ? "" : "?pageToken=" + pageToken;
      final String path = "projects/" + project + "/subscriptions" + query;
      return get("list subscriptions", path, readJson(SubscriptionList.class));
    }
docs: Get a page of Pub/Sub subscriptions in a project using a specified page token.
  
  @param project   The Google Cloud project.
  @param pageToken A token for the page of subscriptions to get.
  @return A future that is completed when this request is completed.