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>

custom/conf/app.example.ini

@@ -2041,6 +2041,8 @@ PATH =
 ;ENABLED = false
 ;; If you want to add authorization, specify a token here
 ;TOKEN =
+;; Enable issue by label metrics; default is false
+;ENABLED_ISSUE_BY_LABEL = false
 
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