feat: Support key filtering position

[miquelbeltran]

According to the documentation:

Supports * at the end of a key to indicate you want to filter any key that contains that key, ie foo_* will filter foo_bar, foo_qux, foo_baz etc

The current code doesn't seem right, as it filters any match of foo_ not only at the start. We would need to fix the implementation so it supports both foo_*, *_foo and *foo*.

Originally posted by @miquelbeltran in #99 (comment)