> based on no closely relevant experience

I don’t want to devolve this to an argument from authority, but - there’s a lot of trade offs to monitoring systems, especially at that scale. Among other things, aggregation takes time at scale, and with enough metrics and numbers coming in, your variance is all over the place. A core fact about distributed systems at this scale is that something is always broken somewhere in the stack - the law of averages demands it, and so if you’re going to do an all-fire-alarm alert any time part of the system isn’t working, you’ve got alarms going off 24/7. Actually detecting that an actual incident is actually happening on a machine of the size and complexity we’re talking about within 5 minutes is absolutely fantastic.

Is your external monitor working? How many checks failed, in what order? Across how many different regions or systems? Was it a transient failure? How many times do you retry, and at what cadence? Do you push your success or failure metrics? Do you pull? What if your metrics don’t make it back? How long do you wait before considering it a problem? What other checks do you run, and how long do those take? What kind of latency is acceptable for checks like that? How many false alarms are you willing to accept, and at what cadence?