Python: expose 'channel' as a property of the transport.

Permits app developers to e.g. perform explicit 'close' on channels for
dynamically-created clients.

See: googleapis/google-cloud-python#5523.

Closes #2337.

src/main/resources/com/google/api/codegen/py/transport.snip

@@ -52,6 +52,8 @@
                     credentials=credentials,
                 )
 
+            self._channel = channel
+
             @# gRPC uses objects called "stubs" that are bound to the
             @# channel and provide a basic method for each RPC.
             self._stubs = {
@@ -92,6 +94,15 @@
                 scopes=cls._OAUTH_SCOPES,
             )
 
+        @@property
+        def channel(self):
+            """The gRPC channel used by the transport.
+
+            Returns:
+                grpc.Channel: A gRPC channel object.
+            """
+            return self._channel
+
         # ----- RPC CALLS -----
         @join apiMethod : api.apiMethods on BREAK.add(BREAK)
             @@property

src/test/java/com/google/api/codegen/gapic/testdata/py/python_library.baseline

@@ -3527,6 +3527,8 @@ class LibraryServiceGrpcTransport(object):
                 credentials=credentials,
             )
 
+        self._channel = channel
+
         # gRPC uses objects called "stubs" that are bound to the
         # channel and provide a basic method for each RPC.
         self._stubs = {
@@ -3563,6 +3565,15 @@ class LibraryServiceGrpcTransport(object):
             scopes=cls._OAUTH_SCOPES,
         )
 
+    @property
+    def channel(self):
+        """The gRPC channel used by the transport.
+
+        Returns:
+            grpc.Channel: A gRPC channel object.
+        """
+        return self._channel
+
     @property
     def create_shelf(self):
         """Return the gRPC stub for {$apiMethod.name}.

src/test/java/com/google/api/codegen/gapic/testdata/py/python_multiple_services.baseline

@@ -1352,6 +1352,8 @@ class DecrementerServiceGrpcTransport(object):
                 credentials=credentials,
             )
 
+        self._channel = channel
+
         # gRPC uses objects called "stubs" that are bound to the
         # channel and provide a basic method for each RPC.
         self._stubs = {
@@ -1383,6 +1385,15 @@ class DecrementerServiceGrpcTransport(object):
             scopes=cls._OAUTH_SCOPES,
         )
 
+    @property
+    def channel(self):
+        """The gRPC channel used by the transport.
+
+        Returns:
+            grpc.Channel: A gRPC channel object.
+        """
+        return self._channel
+
     @property
     def decrement(self):
         """Return the gRPC stub for {$apiMethod.name}.
@@ -1461,6 +1472,8 @@ class IncrementerServiceGrpcTransport(object):
                 credentials=credentials,
             )
 
+        self._channel = channel
+
         # gRPC uses objects called "stubs" that are bound to the
         # channel and provide a basic method for each RPC.
         self._stubs = {
@@ -1492,6 +1505,15 @@ class IncrementerServiceGrpcTransport(object):
             scopes=cls._OAUTH_SCOPES,
         )
 
+    @property
+    def channel(self):
+        """The gRPC channel used by the transport.
+
+        Returns:
+            grpc.Channel: A gRPC channel object.
+        """
+        return self._channel
+
     @property
     def increment(self):
         """Return the gRPC stub for {$apiMethod.name}.

src/test/java/com/google/api/codegen/gapic/testdata/py/python_no_path_templates.baseline

@@ -1069,6 +1069,8 @@ class NoTemplatesApiServiceGrpcTransport(object):
                 credentials=credentials,
             )
 
+        self._channel = channel
+
         # gRPC uses objects called "stubs" that are bound to the
         # channel and provide a basic method for each RPC.
         self._stubs = {
@@ -1100,6 +1102,15 @@ class NoTemplatesApiServiceGrpcTransport(object):
             scopes=cls._OAUTH_SCOPES,
         )
 
+    @property
+    def channel(self):
+        """The gRPC channel used by the transport.
+
+        Returns:
+            grpc.Channel: A gRPC channel object.
+        """
+        return self._channel
+
     @property
     def increment(self):
         """Return the gRPC stub for {$apiMethod.name}.