Add metrics to get issues by label (#17201)

Add metrics to get issues by label (#17201)

* Add metrics to get issues by label

* Add comment on IssueByLabelCount

* Code review - Unify "AS" in SQL (#17201)

* Code review - Remove useless join (#17201)

* Code review - Disable issue_by_label by default in settings (#17201)

* use e

* restore empty line

* update docs

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: techknowlogick <matti@mdranta.net>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>