gce: Use user-data instead of startup-script metadata key

[hakman]

See: hashicorp/terraform-provider-google#1816

Also: https://cloudinit.readthedocs.io/en/23.2.1/reference/datasources/gce.html

The GCE datasource gets its data from the internal compute metadata server. Metadata can be queried at the URL http://metadata.google.internal/computeMetadata/v1/ from within an instance. For more information see the GCE metadata docs.
user-data and user-data-encoding can be provided to cloud-init by setting those custom metadata keys for an instance.

Closes #15589

[hakman]

/cc @justinsb
/assign @justinsb

[hakman]

/kind office-hours

[justinsb]

Thanks @hakman - verified that is now generally supported, even on COS which I think was the problem a few years ago.

/approve
/lgtm

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[hakman]

Thanks @hakman - verified that is now generally supported, even on COS which I think was the problem a few years ago.

Thanks for checking! This will make some people happy. 😄