feat: add support for context manager in client

gapic/templates/%namespace/%name_%version/%sub/services/%service/client.py.j2

@@ -480,6 +480,17 @@ class {{ service.client_name }}(metaclass={{ service.client_name }}Meta):
         {{ "\n" }}
     {% endfor %}
 
+    def __enter__(self):
+        self.transport.__enter__()
+        return self
+
+    def __exit__(self, type, value, traceback):
+        """Releases underlying transport's resources.
+
+        WARNING: This makes the transport unusable.
+        """
+        return self.transport.__exit__(type, value, traceback)
+
     {% if opts.add_iam_methods %}
     def set_iam_policy(
         self,

gapic/templates/%namespace/%name_%version/%sub/services/%service/transports/base.py.j2

@@ -183,6 +183,11 @@ class {{ service.name }}Transport(abc.ABC):
             {% endfor %} {# precomputed wrappers loop #}
         }
 
+    def __enter__(self):
+        raise NotImplementedError()
+
+    def __exit__(self, type, value, traceback):
+        raise NotImplementedError()
 
     {% if service.has_lro %}
 

gapic/templates/%namespace/%name_%version/%sub/services/%service/transports/grpc.py.j2

@@ -224,6 +224,15 @@ class {{ service.name }}GrpcTransport({{ service.name }}Transport):
         """Return the channel designed to connect to this service.
         """
         return self._grpc_channel
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, type, value, traceback):
+        """Closes underlying gRPC channel.
+        """
+        self.grpc_channel.close()
+
     {% if service.has_lro %}
 
     @property

gapic/templates/%namespace/%name_%version/%sub/services/%service/transports/rest.py.j2

@@ -133,6 +133,14 @@ class {{ service.name }}RestTransport({{ service.name }}Transport):
     {% for method in service.methods.values() %}
     {% if method.http_opt %}
 
+    def __enter__(self):
+        return self
+
+    def __exit__(self, type, value, traceback):
+        """Closes underlying HTTP session.
+        """
+        self._session.close()
+
     def {{ method.name|snake_case }}(self,
             request: {{ method.input.ident }}, *,
             retry: retries.Retry = gapic_v1.method.DEFAULT,

tests/system/test_client_context_manager.py

@@ -0,0 +1,34 @@
+# Copyright 2021 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+
+import pytest
+
+from conftest import construct_client, EchoClient
+
+
+def test_client():
+    with construct_client(EchoClient, False) as c:
+        resp = c.echo({
+            'content': 'hello'
+        })
+        assert resp.content == 'hello'
+
+
+def test_client_destroyed(echo):
+    echo.__exit__(None, None, None)
+    with pytest.raises(ValueError):
+        echo.echo({
+            'content': 'hello'
+        })