Address Synchronization Comment from #26405 (#26410)

* Fix metadata plugin for concurrent invocations

* Disable broad-except lint error

* Use WaitGroup instead of threading.Event

* Fix typo

* Use only a single channel

* Yapf

src/python/grpcio_tests/tests/unit/_contextvars_propagation_test.py

@@ -127,15 +127,16 @@ class ContextVarsPropagationTest(unittest.TestCase):
                 test_call_credentials, "test call credentials")
             composite_credentials = grpc.composite_channel_credentials(
                 local_credentials, call_credentials)
-            start_event = threading.Event()
+            wait_group = test_common.WaitGroup(_THREAD_COUNT)
 
             def _run_on_thread(exception_queue):
                 try:
-                    for _ in range(_THREAD_COUNT):
-                        with grpc.secure_channel(
-                                target, composite_credentials) as channel:
-                            start_event.wait()
+                    with grpc.secure_channel(target,
+                                             composite_credentials) as channel:
                         stub = channel.unary_unary(_UNARY_UNARY)
+                        wait_group.done()
+                        wait_group.wait()
+                        for i in range(_RPC_COUNT):
                             response = stub(_REQUEST, wait_for_ready=True)
                             self.assertEqual(_REQUEST, response)
                 except Exception as e:  # pylint: disable=broad-except
@@ -148,7 +149,7 @@ class ContextVarsPropagationTest(unittest.TestCase):
                 thread.setDaemon(True)
                 thread.start()
                 threads.append((thread, q))
-            start_event.set()
+
             for thread, q in threads:
                 thread.join()
                 if not q.empty():