fix: Make external_account resistant to string type 'expires_in' responses from non-compliant services (#1379)

Make external_account resistant to string type 'expires_in' responses from non-compliant services.

This is to complete [#1208](https://togithub.com/googleapis/google-auth-library-python/pull/1208/).

A client run into this error.

Author: duzun

google/auth/external_account.py

@@ -385,7 +385,14 @@ def refresh(self, request):
                 additional_headers=additional_headers,
             )
             self.token = response_data.get("access_token")
-            lifetime = datetime.timedelta(seconds=response_data.get("expires_in"))
+            expires_in = response_data.get("expires_in")
+            # Some services do not respect the OAUTH2.0 RFC and send expires_in as a
+            # JSON String.
+            if isinstance(expires_in, str):
+                expires_in = int(expires_in)
+
+            lifetime = datetime.timedelta(seconds=expires_in)
+
             self.expiry = now + lifetime
 
     @_helpers.copy_docstring(credentials.CredentialsWithQuotaProject)

system_tests/secrets.tar.enc

@@ -625,19 +625,20 @@ def test_is_workforce_pool_with_users_and_impersonation(self, audience):
         # Even though impersonation is used, is_workforce_pool should still return True.
         assert credentials.is_workforce_pool is True
 
+    @pytest.mark.parametrize("mock_expires_in", [2800, "2800"])
     @mock.patch(
         "google.auth.metrics.python_and_auth_lib_version",
         return_value=LANG_LIBRARY_METRICS_HEADER_VALUE,
     )
     @mock.patch("google.auth._helpers.utcnow", return_value=datetime.datetime.min)
     def test_refresh_without_client_auth_success(
-        self, unused_utcnow, mock_auth_lib_value
+        self, unused_utcnow, mock_auth_lib_value, mock_expires_in
     ):
         response = self.SUCCESS_RESPONSE.copy()
         # Test custom expiration to confirm expiry is set correctly.
-        response["expires_in"] = 2800
+        response["expires_in"] = mock_expires_in
         expected_expiry = datetime.datetime.min + datetime.timedelta(
-            seconds=response["expires_in"]
+            seconds=int(mock_expires_in)
         )
         headers = {
             "Content-Type": "application/x-www-form-urlencoded",