Add _metadata.get_project_id.
diff --git a/google/auth/_default.py b/google/auth/_default.py
index 74b96b3..356780b 100644
--- a/google/auth/_default.py
+++ b/google/auth/_default.py
@@ -172,7 +172,7 @@
     if _metadata.ping(request=request):
         # Get the project ID.
         try:
-            project_id = _metadata.get(request, 'project/project-id')
+            project_id = _metadata.get_project_id(request=request)
         except exceptions.TransportError:
             _LOGGER.warning(
                 'No project ID could be determined from the Compute Engine '