I want a WebApp monitoring tool for DevOps. Here is how it should work

I'm a developer. I write web apps and run them on linux servers. I keep an eye on them to make sure all apps are running smoothly. But, as the number of apps and servers grows, manual monitoring takes more and more time, so I need an automated web app monitoring tool.

I will ask the obvious question: Why not make it yourself? You have a clear set of requirements and you are a developer.