feat(genai): count token samples

[Guiners]

Description

Fixes #

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Checklist

• I have followed guidelines from CONTRIBUTING.MD and Samples Style Guide
• Tests pass: npm test (see Testing)
• Lint pass: npm run lint (see Style)
• Required CI tests pass (see CI testing)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This pull request is from a branch created directly off of GoogleCloudPlatform/nodejs-docs-samples. Not a fork.
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new sample directory, and I created GitHub Actions workflow for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

Note: Any check with (dev), (experimental), or (legacy) can be ignored and should not block your PR from merging (see CI testing).

[gemini-code-assist]

Summary of Changes

Hello @Guiners, 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 introduces new Node.js samples for the Google GenAI SDK, specifically demonstrating how to count tokens for text inputs using different methods. It also updates an existing sample to use a newer model version, ensuring the examples are current and functional.

Highlights

• New Token Counting Sample (computeTokens): Added a new sample (counttoken-compute-with-txt.js) to showcase token counting for text using the computeTokens method.
• New Token Usage Metadata Sample: Introduced another new sample (counttoken-resp-with-txt.js) demonstrating how to retrieve token usage metadata from the generateContent response.
• Model Version Update: Updated the counttoken-with-txt.js sample to utilize the gemini-2.5-flash model, ensuring the samples use the latest available models.
• New Test Files: Included new test files for the newly added token counting samples to ensure their functionality.

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[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• genai/count-tokens/counttoken-compute-with-txt.js:17, tag googlegenaisdk_counttoken_compute_with_txt
• genai/count-tokens/counttoken-resp-with-txt.js:17, tag googlegenaisdk_counttoken_resp_with_txt

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[gemini-code-assist]

Code Review

This pull request introduces new samples for counting tokens with the GenAI SDK. The changes look good overall, but I have a few suggestions to improve the new files. I've pointed out an incorrect region tag in one of the samples. For the new tests, I've suggested removing the unnecessary async keyword from the describe blocks and strengthening the assertions to make the tests more robust. These changes will improve the maintainability and correctness of the new samples.

[msampathkumar]

Missing log responses.

Check https://github.com/GoogleCloudPlatform/python-docs-samples/blob/c79f7d5b123cf10a19e720e52a3aa94b45b4feb3/genai/count_tokens/counttoken_compute_with_txt.py#L27-L34