WL#9629: Enable PFS MDL instrumentation by Default

MDL instrumentation is useful for understanding lurking locking problems, which 
are not clear in SHOW PROCESSLIST etc.

We can create some nice views of MDL in SYS once it is enabled:
performance-schema-instrument='wait/lock/metadata/sql/%=ON'

For example:
https://bugs.mysql.com/bug.php?id=80823

User Documentation
==================

* https://dev.mysql.com/doc/relnotes/mysql/8.0/en/news-8-0-2.html
* https://dev.mysql.com/doc/refman/8.0/en/metadata-locks-table.html