Netdata

https://www.netdata.cloud/

Infrastructure monitoring tool. It monitors status of servers, such as CPU, RAM usage (and usage by each app) and much more.

Open Source

Many ways to deploy, and deployment is super easy.

There is

• Linux One-Liner
• Docker

and much more.

Netdata can run locally, or linked to the cloud so that you can monitory many instances at once, and receive alerts. Cloud is free for basic functions. Extra functionalities come with a cost.