metrics: expose `total_developers` code metric in the website

[ccerv1]

What is it?

Actions:

• Show the all-time count of any Github user who has made a commit to the main branch of a project repo
• Add better copy to the website to indicate that contributors includes activities like opening issues, PRs, etc and not just code contributions See also https://github.com/opensource-observer/oss-directory/issues/447