What MySQL configuration is required to enable performance_schema.variables_by_thread?

I am trying to read the [performance_schema.variables_by_thread](https://dev.mysql.com/doc/refman/8.4/en/performance-schema-system-variable-tables.html) table with the following query, but it always returns an empty set:

mysql> select * from performance_schema.variables_by_thread limit 1;
Empty set (0.00 sec)

This is an active production server with hundreds of connections, so I think it is unlikely to actually be empty. **Is there some configuration flag that must be set to enable this functionality?** MySQL version information:

mysql> select version();
+-----------+
| version() |
+-----------+
| 8.0.35-27 |
+-----------+
1 row in set (0.00 sec)

Also, server greeting string:

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 107143716
Server version: 8.0.35-27 Percona Server (GPL), Release 27, Revision 2f8eeab2