The socket marked with wildcards (*) is like an array. It can occur multiple times.
The solution was just to append more lines with socket=... to specify all instances.
What is not working with this semantics:
- alias for each instance
- different user/password for each instance
if you need such thing you need to modify the mk_mysql plugin file:
checkmk/mk_mysql at master · tribe29/checkmk (github.com)