[RFE] Add support for GitHub Actions :octocat:

[jamacku]

Currently, metadata only works with GitHub Apps but not with GitHub Actions.

At the moment, metadata requires app ID, which is unavailable when using probot/adapter-github-actions. Therefore I have been using a slightly modified version of metadata embedded into my Action - link It is using the name of GitHub action instead of app ID. It isn't probably the perfect identifier, but it has worked for me so far :crossed_fingers:.

It would be nice to have native metadata support when using GitHub Action. Thank you.