That is, if you know what to monitor. When you launch an entirely new feature, you often don't know all the ways it will break. And it's not trivial to set up

That is, if you know what to monitor. When you launch an entirely new feature, you often don't know all the ways it will break. And it's not trivial to set up a correct alert for this one. Simple HTTP 5xx threshold wouldn't work because it's high enough to not wake you up whenever cloud provider restarts Postgres, it's too high to catch this. You need either per-endpoint failure rate alerts or something else more clever.