Add Langfuse observability configuration to env.example

- Add Langfuse environment variables
- Include setup instructions
- Support OpenAI compatible APIs
- Enable tracing configuration
- Add cloud/self-host options

env.example

@@ -447,6 +447,17 @@ MEMGRAPH_DATABASE=memgraph
 ### DB specific workspace should not be set, keep for compatible only
 ### MEMGRAPH_WORKSPACE=forced_workspace_name
 
+###########################################################
+### Langfuse Observability Configuration
+### Only works with LLM provided by OpenAI compatible API
+### Install with: pip install lightrag-hku[observability]
+### Sign up at: https://cloud.langfuse.com or self-host
+###########################################################
+# LANGFUSE_SECRET_KEY=""
+# LANGFUSE_PUBLIC_KEY=""
+# LANGFUSE_HOST="https://cloud.langfuse.com"  # 或您的自托管实例地址
+# LANGFUSE_ENABLE_TRACE=true
+
 ############################
 ### Evaluation Configuration
 ############################