Hi… It’s listed as being unavailable, if I’m reading the official Fly MPG extensions doc correctly:
https://fly.io/docs/mpg/extensions/
Other people have requested it, though…
https://community.fly.io/t/mpg-extensions-management-is-here/25551/4
The fine print in the official doc recommends an alternative:
Consider using an advanced version of
pg_stat_statements-pg_stat_monitor
Hope this helps a little!
Aside: Fly MPG does include Support, so, for best results, you can ask them directly, instead of rolling the dice here in the community forum, 
.