Rename default branch to main

In line with [Prometheus][1], I'd like to rename the default branch from `master` to `main`. This will be done _after_ all PRs are accepted/closed. People who forked the repo may need to [update their fork][2].

[1]: https://github.com/prometheus/prometheus/issues/8527
[2]: https://www.codingnagger.com/2021/09/26/easily-rename-your-git-default-branch-from-main-to-master/