I spent enough time ~15 years back to find an external monitoring service that did not run on AWS and looked like a sustainable business instead of a VC fueled acquisition target - for our belts-n-braces secondary monitoring tool since it's not smart to trust CloudWatch to be able to send notifications when it's AWS's shit that's down.
Sadly while I still use that tool a couple of jobs/companies later - I no longer recommend it because it migrated to AWS a few years back.
(For now, my out-of-AWS monitoring tool is a bunch of cron jobs running on a collections of various inexpensive vpses and my and other dev's home machines.)
Interestingly, the reason I originally looked for and started using it was an unapproved "shadow IT" response to an in-house Nagios setup that was configured and managed so badly it had _way_ more downtime than any of the services I'd get shouted about at if customers noticed them down before we did...
(No disrespect to Nagios, I'm sure a competently managed installation is capable of being way better than what I had to put up with.)
Sadly while I still use that tool a couple of jobs/companies later - I no longer recommend it because it migrated to AWS a few years back.
(For now, my out-of-AWS monitoring tool is a bunch of cron jobs running on a collections of various inexpensive vpses and my and other dev's home machines.)