Override resourcemanager_v3::ProjectsClient Endpoint Configuration 

  // This service supports specifying a regional or locational endpoint prefix
  // when creating the ProjectsConnection.
  // For example, to connect to
  // "europe-central2-cloudresourcemanager.googleapis.com":
  auto client = google::cloud::resourcemanager_v3::ProjectsClient(
      google::cloud::resourcemanager_v3::MakeProjectsConnection(
          "europe-central2"));

  // This configuration is common with Private Google Access:
  //     https://cloud.google.com/vpc/docs/private-google-access
  auto options = google::cloud::Options{}.set<google::cloud::EndpointOption>(
      "private.googleapis.com");
  auto vpc_client = google::cloud::resourcemanager_v3::ProjectsClient(
      google::cloud::resourcemanager_v3::MakeProjectsConnection(options));