Allow Cloud Run metadata block to be optional

[chrisst]

The create request doesn't actually require this field. Unfortunately this probably should have been a purely computed field, but I'll file a 4.0 change for this.

Release Note Template for Downstream PRs (will be copied)

`cloudrun`: Stop requiring the root `metadata` block for `google_cloud_run_service`.

[modular-magician]

Hi! I'm the modular magician, I work on Magic Modules.
I see that this PR has already had some downstream PRs generated. Any open downstreams are already updated to your most recent commit, d3ae91b.

Pull request statuses

No diff detected in Ansible.
No diff detected in Inspec.

New Pull Requests

I built this PR into one or more new PRs on other repositories, and when those are closed, this PR will also be merged and closed.
depends: hashicorp/terraform-provider-google-beta#1478
depends: GoogleCloudPlatform/terraform-google-conversion#279
depends: hashicorp/terraform-provider-google#5094

[danawillow]

Namespace was already being inferred from the project, right? This just made it so you don't have to set the metadata block at all.

[modular-magician]

Hi! I'm the modular magician, I work on Magic Modules.
I see that this PR has already had some downstream PRs generated. Any open downstreams are already updated to your most recent commit, f82e4e9.

Pull request statuses

terraform-provider-google-beta already has an open PR.
terraform-google-conversion already has an open PR.
terraform-provider-google already has an open PR.
No diff detected in Ansible.
No diff detected in Inspec.

New Pull Requests

I didn't open any new pull requests because of this PR.