fix: auth fetch token from default endpoint (#1779)

Author: harkamaljot

google/auth/compute_engine/credentials.py

@@ -124,8 +124,9 @@ def refresh(self, request):
         scopes = self._scopes if self._scopes is not None else self._default_scopes
         try:
             self._retrieve_info(request)
+            # Always fetch token with default service account email.
             self.token, self.expiry = _metadata.get_service_account_token(
-                request, service_account=self._service_account_email, scopes=scopes
+                request, service_account="default", scopes=scopes
             )
         except exceptions.TransportError as caught_exc:
             new_exc = exceptions.RefreshError(caught_exc)

tests/compute_engine/test_credentials.py

@@ -501,7 +501,7 @@ def test_with_target_audience_integration(self):
         responses.add(
             responses.GET,
             "http://metadata.google.internal/computeMetadata/v1/instance/"
-            "service-accounts/service-account@example.com/token",
+            "service-accounts/default/token",
             status=200,
             content_type="application/json",
             json={
@@ -659,7 +659,7 @@ def test_with_quota_project_integration(self):
         responses.add(
             responses.GET,
             "http://metadata.google.internal/computeMetadata/v1/instance/"
-            "service-accounts/service-account@example.com/token",
+            "service-accounts/default/token",
             status=200,
             content_type="application/json",
             json={