feat: add 'Client.close' (#100)

FBO use with 'contextlib.closing'.

Closes #64.

google/cloud/client.py

@@ -214,6 +214,21 @@ def _http(self):
             self._http_internal.configure_mtls_channel(self._client_cert_source)
         return self._http_internal
 
+    def close(self):
+        """Clean up transport, if set.
+
+        Suggested use:
+
+        .. code-block:: python
+
+           import contextlib
+
+           with contextlib.closing(client):  # closes on exit
+               do_something_with(client)
+        """
+        if self._http_internal is not None:
+            self._http_internal.close()
+
 
 class _ClientProjectMixin(object):
     """Mixin to allow setting the project on the client.

tests/unit/test_client.py

@@ -205,6 +205,21 @@ def test_from_service_account_json(self):
         file_open.assert_called_once_with(mock.sentinel.filename, "r", encoding="utf-8")
         constructor.assert_called_once_with(info)
 
+    def test_close_w__http_internal_none(self):
+        credentials = _make_credentials()
+        client_obj = self._make_one(credentials=credentials, _http=None)
+
+        client_obj.close()  # noraise
+
+    def test_close_w__http_internal_set(self):
+        credentials = _make_credentials()
+        http = mock.Mock(spec=["close"])
+        client_obj = self._make_one(credentials=credentials, _http=http)
+
+        client_obj.close()
+
+        http.close.assert_called_once_with()
+
 
 class Test_ClientProjectMixin(unittest.TestCase):
     @staticmethod