Add discovery service to stable APIs

generated/googleapis/test/adexchangebuyer/v1_3_test.dart

 library googleapis.adexchangebuyer.v1_3.test;
 
 import "dart:core" as core;
 import "dart:collection" as collection;
 import "dart:async" as async;
 import "dart:convert" as convert;
 
 import 'package:http/http.dart' as http;
 import 'package:http/testing.dart' as http_testing;
 import 'package:unittest/unittest.dart' as unittest;
@@ skipping 2244 matching lines @@
       res.update(arg_request, arg_accountId, arg_configId).then(unittest.expectAsync(((api.PretargetingConfig response) {
         checkPretargetingConfig(response);
       })));
     });
 
   });
 
 
 }