docs: add langfuse observability guide (#1047)

Co-authored-by: angiejones <[email protected]>

documentation/docs/tutorials/langfuse.md

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+---
+description: Integrate Goose with Langfuse to observe performance
+---
+
+# Observability with Langfuse
+
+This tutorial covers how to integrate Goose with Langfuse to monitor your Goose requests and understand how the agent is performing.
+
+## What is Langfuse
+
+[Langfuse](https://langfuse.com/) is an [open-source](https://github.com/langfuse/langfuse) LLM engineering platform that enables teams to collaboratively monitor, evaluate, and debug their LLM applications.
+
+
+## Set up Langfuse
+
+Sign up for Langfuse Cloud [here](https://cloud.langfuse.com) or self-host Langfuse [Docker Compose](https://langfuse.com/self-hosting/local) to get your Langfuse API keys.
+
+## Configure Goose to Connect to Langfuse
+
+Set the environment variables so that Goose (written in Rust) can connect to the Langfuse server.
+
+```bash
+export LANGFUSE_INIT_PROJECT_PUBLIC_KEY=pk-lf-...
+export LANGFUSE_INIT_PROJECT_SECRET_KEY=sk-lf-...
+export LANGFUSE_HOST=https://cloud.langfuse.com # EU data region 🇪🇺
+
+# https://us.cloud.langfuse.com if you're using the US region 🇺🇸
+# https://localhost:3000 if you're self-hosting
+```
+
+## Run Goose with Langfuse Integration
+
+Now, you can run Goose and monitor your AI requests and actions through Langfuse.
+
+With Goose running and the environment variables set, Langfuse will start capturing traces of your Goose activities.
+
+_[Example trace (public) in Langfuse](https://cloud.langfuse.com/project/cloramnkj0002jz088vzn1ja4/traces/cea4ed38-0c44-4b0a-8c20-4b0b6b9e8d73?timestamp=2025-01-31T15%3A52%3A30.362Z&observation=7c8e5807-3c29-4c28-9c6f-7d7427be401f)_
+
+![Goose trace in Langfuse](https://langfuse.com//images/docs/goose-integration/goose-example-trace.png)
+