Uptime Kuma
Uptime Kuma is open source, self-hosted uptime monitoring with status pages. It runs on Node.js with a server or Docker of its own, not on fortrabbit, and monitors fortrabbit sites from there.
About Uptime Kuma
Uptime Kuma checks HTTP, keywords, ports, DNS and more, with notifications to many chat services and public status pages. The source is on GitHub: louislam/uptime-kuma.
Pricing
Free and open source. It needs a server of its own.
Integration
Uptime Kuma is a Node.js app and runs on a server with Docker, not on fortrabbit. Monitor the fortrabbit domains from there, ideally from another provider than the one hosting the website.