Hello Andreas,
thanks a lot for your research!!
I’m not sure if I got you right.
I checked the lines in my metric file: /opt/omd/versions/1.5.0p24.cre/share/check_mk/web/plugins/metrics/check_mk.py
They are exact matching the green lines in the commit you linked to.
What I did not undestand is your last sentence, do I have to change something in that file altought there are the same as in the commit?
Sorry for my inability 
Best regards
Henry
btw. I meanwhile also tried jumping to 1.6.0p8 with exactly the same result, so maybe it is a wanted change in design and not a bug?