Part of the problem is that computing the downtime at a higher resolution, in a way that scales from low-traffic users (the quoted minimum of 60 requests per mi

Part of the problem is that computing the downtime at a higher resolution, in a way that scales from low-traffic users (the quoted minimum of 60 requests per minute is also not a coincidence) to very large ones, is not trivial. You have to consider the rate at which the raw data is collected from the replicas ---and the jitter across all of them!-- then the rate at which that data is aggregated, and so on. (The above assumes that Cloud Spanner uses Borgmon for monitoring.) Any hiccups in the collection itself will result in less noise if you look at 5m vs e.g. 1m. You could increase the sampling rate, but then you're making your DB replicas spend more precious cycles on work that is not serving queries or replicating data.