Sorry if my answer will not provide a solution or recommend a tool. I just want to make you aware of couple of things.
Monitoring is too general you need specify what are you trying to check. This will help you select a software/tool or even using multiple.
aspects of Monitoring (or Observability):
Audit Logs (audit logs is used for specific events/actions that is important from security or compliance perspective)
Health checks (related to up time & wether the service is currently running normally)
Logs (Applications logs, Server Logs, Database logs........etc). which shows exceptions to be able to track bugs, or even crashes in your Database or web servers