Fix flaky ArgumentException in perfdiag tests
Fix flaky ArgumentException in perfdiag tests

This fixes a bug where the out-scoped dst_obj_metadata
object would not be valid in the Compose call and result
in a dst_obj_metadata with no name/bucket.

GoogleCloudPlatform/gsutil,GoogleCloudPlatform/gsutil,fishjord/gsutil,BrandonY/gsutil