Override the default endpoint

 namespace spanner = ::google::cloud::spanner;
  [](std::string const& project_id, std::string const& instance_id,
     std::string const& database_id) {
    auto options = google::cloud::Options{}.set<google::cloud::EndpointOption>(
        "private.googleapis.com");
    return spanner::Client(spanner::MakeConnection(
        spanner::Database(project_id, instance_id, database_id), options));
  }