Add discovery service to stable APIs

generated/googleapis/test/content/v2_test.dart

 library googleapis.content.v2.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 4430 matching lines @@
       res.list(arg_merchantId, maxResults: arg_maxResults, pageToken: arg_pageToken).then(unittest.expectAsync(((api.ProductstatusesListResponse response) {
         checkProductstatusesListResponse(response);
       })));
     });
 
   });
 
 
 }