[llm direct] Handle immutability for spec.metricDescriptor.labels field

[gemmahou]

Checklist

• I did not find a related open enhancement request.
• I understand that enhancement requests filed in the GitHub repository are by default low priority.
• If this request is time-sensitive, I have submitted a corresponding issue with GCP support.

Describe the feature or resource

Per the API design, it's not allowed to modify any existing spec.metricDescriptor.labels fields, but it's possible to add/delete labels.

Can we handle the blocking of updates on labels fields earlier, either in our webhook or direct controller, to avoid sending HTTP requests to the GCP API? This would reduce the number of requests sent to GCP.

Additional information

No response

Importance

No response

[acpana]

cc @acpana @yuwenma