Support custom metadata labels for partner models on Vertex AI #6793

Is your feature request related to a problem? Please describe.
Vertex AI's custom metadata labels only work for Google models. We need billing labels on partner model calls (Anthropic) for cost attribution and there's no supported way to do it.

Describe the solution you'd like
Extend the existing labels field to work with partner model API calls so they appear in Cloud Billing reports.

Describe alternatives you've considered
Client-side HTTP intercept to manually track token usage, but this doesn't integrate with GCP billing dashboards.

Additional context
Labels docs: https://docs.cloud.google.com/vertex-ai/generative-ai/docs/multimodal/add-labels-to-api-calls
Community thread: https://community.latenode.com/t/adding-custom-metadata-labels-to-anthropic-model-requests-on-gcp-vertex-ai/37152/5