Add open_issues}/%closed_issues for badges
Similar to gitlab-ce#56995 it'd be nice to have an open_issues/closed_issues tag for badges, especially if there's a way to indicate issues for a specific milestone.
Something like this:
`https://gitlab.com/%{project_path}/%{project_id}/badges/open_issues.svg` for overall
`https://gitlab.com/%{project_path}/%{project_id}/badges/1.0.0/open_issues.svg` for issues tagged for a specific milestone
## Implementation guide
TBD
issue