Override the authentication configuration

 namespace spanner = ::google::cloud::spanner;
  [](std::string const& project_id, std::string const& instance_id,
     std::string const& database_id, std::string const& keyfile) {
    auto is = std::ifstream(keyfile);
    is.exceptions(std::ios::badbit);
    auto contents = std::string(std::istreambuf_iterator<char>(is.rdbuf()), {});
    auto options =
        google::cloud::Options{}.set<google::cloud::UnifiedCredentialsOption>(
            google::cloud::MakeServiceAccountCredentials(contents));
    return spanner::Client(spanner::MakeConnection(
        spanner::Database(project_id, instance_id, database_id), options));
  }