As you already found in our documentation - DevOps metrics are available if you use BitBucket, but those metrics are not available yet for GitHub.
For GitHub - we have this integration in our backlog but currently without any estimates on when it could be implemented. I added your vote to that ticket and I will update this post if anything changes.
You may want to look into the option of Git commit log analysis in a data cube, separate from Jira. For example, you can upload a Git commit log as a file source to eazyBI and import it into a separate cube. eazyBI recognizes the Git commit log data structure.
More on this in our documentation: Git commit log