Custom dynamic version provider for Python projects

Question

In a Python project that uses pyproject.toml (and setuptools), I want to have a custom version number provider. I want it to work like setuptools_scm, but use my own logic to determine the version (and preferably other fields such as description and author). I know it can be accomplished by using dynamic metadata, but I would like it to work just by including the module in [build-system]. How do I do that? How does setuptools knows where to get the metadata from?