feat(genai): Adding 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).

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• genai/count-tokens/counttoken-localtokenizer-compute-with-txt.js:17, tag googlegenaisdk_counttoken_localtokenizer_compute_with_txt
• genai/count-tokens/counttoken-localtokenizer-with-txt.js:17, tag googlegenaisdk_counttoken_localtokenizer_with_txt

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[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 enhances the Generative AI samples by adding new functionalities for token counting. It provides developers with practical examples of how to use the Google Generative AI SDK to either retrieve a detailed breakdown of tokens or simply get the total token count for a given text input. These additions are crucial for understanding and managing the token usage of AI models.

Highlights

• New Token Counting Samples: Introduced two new Node.js samples demonstrating how to count tokens using the Google Generative AI SDK for text prompts.
• Detailed Token Information: One sample, counttoken-localtokenizer-compute-with-txt.js, showcases the client.models.computeTokens method, which provides detailed information about the tokens in a given text input, including token IDs and the tokens themselves.
• Total Token Count: Another sample, counttoken-localtokenizer-with-txt.js, demonstrates the client.models.countTokens method, which returns the total number of tokens for a text prompt.
• Unit Tests Added: Corresponding unit tests have been added for both new token counting samples to ensure their correct functionality and output.

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[gemini-code-assist]

Code Review

This pull request adds two new samples for counting tokens with genai, along with their tests. The changes look good, but I have a few suggestions to improve consistency, correctness, and robustness.

My main feedback points are:

• Adopting camelCase for function names for consistency.
• Modifying sample functions to accept project and location parameters, which is a common pattern in this repository and makes them more testable.
• Adding validation for the projectId.
• Correcting a misleading test description and an example output comment.

I've left detailed comments with code suggestions for each point.