Support GitLab-compatible Cobertura format in MTP natively

Hello 👋,

I’m currently using GitLab, which supports displaying code coverage information directly inside the diff view of a merge request. It relies on the Cobertura format for this feature.

Previously, when I was using coverlet, this worked perfectly out of the box. However, after switching to Microsoft.Testing.Platform and the Microsoft.Testing.Extensions.CodeCoverage package, the coverage report stopped working in GitLab.

After some experimentation, I was able to make it work again by manually tweaking the generated Cobertura XML file:

dotnet run -c Release -- --report-junit --coverage --coverage-output-format cobertura --coverage-output myapp.cobertura.xml
sed -i "s@filename=\"$CI_PROJECT_DIR/@filename=\"@g" bin/Release/net9.0/TestResults/myapp.cobertura.xml
sed -i "s@<packages>@<sources><source>$CI_PROJECT_DIR/</source></sources><packages>@g" bin/Release/net9.0/TestResults/myapp.cobertura.xml

The idea here is:

• Replace all absolute paths in the nodes with relative paths.
• Add a node containing the base path.

This makes GitLab correctly display coverage information in merge requests.
However, having to run these sed commands after each test run is not ideal.

👉 Question:
Is there already a built-in option or configuration to achieve this behavior natively?
And if not, would it be possible to add support for relative paths and sources in the Cobertura output?

Thanks a lot for your work on this project!

Start by running the documented dotnet command with --coverage-output-format cobertura and inspect the generated Cobertura XML. Compare its class filenames and package structure with the relative paths and sources required by GitLab; done means the native output works without the post-processing sed commands and preserves valid coverage data.