mark client_lb_end2end_test as flaky

test/cpp/end2end/BUILD

@@ -438,6 +438,7 @@ grpc_cc_test(
     external_deps = [
         "gtest",
     ],
+    flaky = True,  # TODO(b/151315347)
     tags = ["no_windows"],  # TODO(jtattermusch): fix test on windows
     deps = [
         ":test_service_impl",