Coverage data in GH Actions

[felixschurk]

When looking, what accidentally happened in #3271, e.g. kicking out the python tests, I guess it would make sense to have some coverage data available in the repository, automatically build by GH Actions on every PR. To provide a broad overview and an easy to see metric for incoming PRs.

As I can see, this was available in #1984, but probably got lost when transitioning to GitHub.
Maybe it can also be used when new functionality is added to have some feedback if it is tested or not.

I guess a starting point would be to use Code Coverage Summary actions and instrument the code accordingly and collecting the data over gcovr.
In order not to blow the complete GH Actions maybe it would make sense to run it only after the test were successful and maybe also only in one image (Debian, Ubuntu or Fedora) as the coverage should not change depending on the OS.

[djmitche]

I think that makes a lot of sense, and particularly the intent to have it wait for successful tests. Noticing when coverage drops dramatically would be very useful.

[felixschurk]

I will work on it :)