Add Agenta as LLM observability alternative

[mmabrouk]

Hi! 👋

I'd like to suggest adding Agenta as an alternative to Langfuse for LLM observability and monitoring in the defensive measures section.

What is Agenta?

Agenta is an open-source LLMOps platform that provides:

• LLM Observability & Monitoring: Track and debug LLM applications in production
• Prompt Management: Version control and collaborative prompt engineering
• LLM Evaluation: Both automated and human-in-the-loop evaluation workflows

Why add it?

• Open-source alternative: Like Langfuse, it's fully open-source and self-hostable
• Production monitoring: Specifically designed for observing and evaluating LLM applications in production (fits perfectly with the defensive measures context)
• Established project: MIT licensed, 3k+ stars, actively maintained since April 2023

Change Made

I've added Agenta alongside Langfuse in line 428:

observe them in production (with a framework like [langfuse](https://github.com/langfuse/langfuse) or [agenta](https://github.com/agenta-ai/agenta))

This provides learners with multiple options for LLM observability frameworks when implementing defensive measures for their applications.

Thanks for considering this addition! 🙏