Override BigtableInstanceAdminClient Default Endpoint

  // 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 client = google::cloud::bigtable_admin::BigtableInstanceAdminClient(
      google
::cloud::bigtable_admin::MakeBigtableInstanceAdminConnection(
          options
));