Override BigtableInstanceAdminClient Default Authentication

 [](std::string const& keyfile) { auto is = std::ifstream(keyfile); is.exceptions(std::ios::badbit); // Minimal error handling in examples auto contents = std::string(std::istreambuf_iterator<char>(is.rdbuf()), {}); auto options = google::cloud::Options{}.set<google::cloud::UnifiedCredentialsOption>( google::cloud::MakeServiceAccountCredentials(contents)); return google::cloud::bigtable_admin::BigtableInstanceAdminClient( google::cloud::bigtable_admin::MakeBigtableInstanceAdminConnection( options)); }