This is one of the biggest PITAs with hosted Postgres - you can't just use whatever extensions you want, or indeed whatever version of those extension. Invariably AWS and Azure (presumably GCP too, I haven't used it) have only the most popular extensions available, and invariably woefully outdated versions.
Longer term we're exploring the ability to bring your own and already have some thoughts on how to make that possible.
Now of course your statement about pg_stat_monitor is correct, too. But you missed when you questioned Peter since he was talking about something different.
Fun fact: Peter Geoghegan is a PostgreSQL major contributor per https://www.postgresql.org/community/contributors/, pretty sure he's a committer as well.