alibaba/Sentinel ? reverse-engineered prompt

Build me a Java library and dashboard for protecting microservices from traffic spikes and unstable downstream services.

I want it to let developers wrap a piece of code as a protected resource, then apply rules to control how many requests get through, limit concurrency, do traffic shaping, and trip circuit breaking when needed. It should also record runtime stats like requests, blocked traffic, success, errors, and response time, so I can watch what is happening in real time for one app or a whole cluster. Please include a simple demo app that shows how to set up a rule and see it work, plus a dashboard where I can view metrics and change rules live. If there are common integrations for things like Spring based services or RPC frameworks, wire those in too. Keep it easy to extend later, and if you need current setup details from the docs, look them up online.