docs(hooks): document all three usageMetadata token fields for LLMResponse

[abhay-codes07]

Summary

The hooks reference documents the stable LLMResponse.usageMetadata schema as a single field:

"usageMetadata": { "totalTokenCount": number }

But a BeforeModel/AfterModel hook actually receives three token fields. The hook-facing translator toHookLLMResponse() in packages/core/src/hooks/hookTranslator.ts builds:

usageMetadata: sdkResponse.usageMetadata
  ? {
      promptTokenCount: sdkResponse.usageMetadata.promptTokenCount,
      candidatesTokenCount: sdkResponse.usageMetadata.candidatesTokenCount,
      totalTokenCount: sdkResponse.usageMetadata.totalTokenCount,
    }
  : undefined,

and the exported LLMResponse interface in the same file declares all three. Since this section is explicitly presented as the stable API hooks can rely on, the reference should list every field the hook is handed — otherwise integrators assume only totalTokenCount is available and miss the input/output token breakdown the code already provides.

Change

Expand the usageMetadata schema in docs/hooks/reference.md to match the interface and translator:

"usageMetadata": {
  "promptTokenCount": number,
  "candidatesTokenCount": number,
  "totalTokenCount": number
}

Docs-only change; prettier --check passes.

Fixes #28048

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request updates the hooks reference documentation to accurately reflect the usageMetadata structure provided to BeforeModel and AfterModel hooks. By documenting all three available token fields, it ensures developers have access to the full token breakdown provided by the underlying implementation, preventing confusion regarding available data.

Highlights

• Documentation Update: Updated the LLMResponse.usageMetadata schema in the hooks reference documentation to include promptTokenCount and candidatesTokenCount fields.

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[gemini-code-assist]

Code Review

This pull request updates the documentation in docs/hooks/reference.md to expand the usageMetadata schema definition, adding promptTokenCount and candidatesTokenCount fields alongside the existing totalTokenCount field. There are no review comments, and I have no feedback to provide.

_{Note: Security Review has been skipped due to the limited scope of the PR.}

[github-actions]

📊 PR Size: size/XS

• Lines changed: 6
• Additions: +5
• Deletions: -1
• Files changed: 1