Fix metadata for storage bucket object

[emilymye]

Release Note Template for Downstream PRs (will be copied)

storage: Fixed setting/reading `google_storage_bucket_object`  metadata on API object

Fixes hashicorp/terraform-provider-google#6256

This wasn't caught by the metadata test because the absence of the field in both create and read - resource.TestCheckResourceAttr checks against state, where metadata was never changed from the initial config value. Usually this is fixed by the import tests.

I don't think storage bucket object is out of the range of autogenerating, so I'll probably do that in a follow-up PR. At the very least, we should support import, though I'm not sure if we avoided it because of content/source.

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 2 files changed, 6 insertions(+))
Terraform Beta: Diff ( 2 files changed, 6 insertions(+))