Hi… This may be postgres_exporter, the metrics subprocess:
$ fly pg connect -a db-app-name
> select usename, query from pg_stat_activity where datname = '<database-name>';
usename query
---------- -----------------
flypgadmin SELECT version();
It auto-attaches to all databases—if I understand correctly.
The easiest is to change the DATABASE_URL to match the Heroku-based name, I think.
(In other words, you redefine the “right name” altogether.)
What have you done already in the way of fly pg attach, etc.?