This is how I resolved my issue:
I ssh’d onto the pg server:
> fly ssh console -a chat-pg
then ran a stolon update similar to kurt’s above
> stolonctl update --patch "{\"pgParameters\": {\"shared_preload_libraries\": \"pg_stat_statements\"}}"
afterwards, the config did show as updated:
> fly postgres config view -a chat-pg
Update available 0.0.441 -> 0.0.442.
Run "fly version update" to upgrade.
NAME VALUE UNIT DESCRIPTION PENDING RESTART
...
shared-preload-libraries -> (pg_stat_statements) Lists shared libraries to preload into server. true
...
Some changes are awaiting a restart!
To apply changes, run: `fly postgres restart --app chat-pg`
Then I followed the instructions in that last command to restart the server. Done!