Add missing labels field to google_compute_instance documentation

[modular-magician]

Closes #11774

PR adds labels to the supported fields in the initialize_params block documentation (currently not in the docs)

Description copied from the code:

https://github.com/GoogleCloudPlatform/magic-modules/blob/c2a78c7a9296eaf813677bea9fd8f1469aef43f8/mmv1/third_party/terraform/resources/resource_compute_instance.go.erb#L217

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests). N/A
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know). N/A
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

Derived from GoogleCloudPlatform/magic-modules#6326

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