Add gauge metric for number of self-hosted org runners

[peterhalasz]

Added a new metric for the number of self-hosted organization action runners

• Added a new gauge metric github_actions_num_self_hosted_org_runners
• Extracted the creation of the GithubClient

It's unclear to me whether reporting the total number of runners is enough by itself. I think the number stays the same even if the registered runners are offline... Alternatively I could add a few labels to the metric like name, os, status, busy, etc. (Github API docs) What do you think?

[cpanato]

need a rebase