Add mutex to compute_project_metadata_item

[modular-magician]

This PR resolves issue hashicorp/terraform-provider-google#9119 on the terraform provider. Adding a mutex to the compute_project_metadata_item resource means that the provider will not fight itself when applying updates to the common instance metadata (which use optimistic locking). A longer term solution would be to implement batching.

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Release Note Template for Downstream PRs (will be copied)

compute: added mutex in `google_compute_metadata_item` to reduce retries + quota errors

Derived from GoogleCloudPlatform/magic-modules#4778