I've Worked With Nagios, but Never in the Same Shop

For me, using nagios was my approach to monitoring. And that's great when you have a persistent stateful single note or couple of single notes. But then you outgrow it because now everything's ephemeral. By the time you realize that there's an outage or an issue with a container, the container hasn't existed for 20 minutes. You'd better have good telematry into what's going on and how your application behaves - especially at scale. One in a million events happen multiple times a second depending upon scale.