Add JS/TypeScript cookbooks for Google Gemini tracing (basic + multimodal)

[JK-Amanda-Goo]

Hi Langfuse team! I'm a developer who built a Next.js fashion moodboard app using Google Gemini + Langfuse for agents actions tracing (photo analysis - product search - discover more). While building it I noticed there are no JavaScript cookbooks for native Google Gemini integration.

I'd like to contribute two cookbooks:

1. js_integration_google_gemini.ipynb — basic Gemini tracing in JS, including the flushAsync() pattern for serverless/Next.js and correct model name for cost tracking

2. js_integration_google_gemini_multimodal.ipynb — tracing image-based Gemini calls (multimodal), showing how to capture image token costs and render images in the Langfuse UI

Both examples come from my working app. Happy to follow any format guidelines you have for the JS cookbooks. Let me know if this is welcome!

[jannikmaierhoefer]

Hi @JK-Amanda-Goo, thank you very much! Feel free to open a PR in our docs repository: https://github.com/langfuse/langfuse-docs/tree/main/cookbook Happy to review it.