Support method-level coverage in Cobertura XML files

[thiagocrepaldi]

As per an enlightening discussion with @nedbat on a Discord thread, coverage.py already reports on function and class coverages. This is achieved through HTML reports, in which we can see "Files, Functions, Classes" at the top of the index page.

However, the (Cobertura) XML report doesn't expose the same information. Instead, the coverage.py's xml report will show empty <methods/> entries

My feature request is to populate the the XML report with the function/methods coverage info. A sample XML on how the content should look like can be seen here

This feature would be helpful for rendering tools to easily report on/parse coverage information and possibly fail CI/CD runs when coverage is low for at the method-level coverage.

As a side note - but probably unrelated to the request, is that another project named fun-coverage implemented method-coverage on terminal-only.