chore(vertexai): add region tag for tokencount

[subfuzion]

Description

Ref b/340935855

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Checklist

• Please merge this PR for me once it is approved

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• vertexai/token-count/tokencount.go:20, tag generativeaionvertexai_gemini_token_count

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