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>

modules/setting/setting.go

@@ -390,11 +390,13 @@ var (
 
 	// Metrics settings
 	Metrics = struct {
-		Enabled bool
-		Token   string
+		Enabled             bool
+		Token               string
+		EnabledIssueByLabel bool
 	}{
-		Enabled: false,
-		Token:   "",
+		Enabled:             false,
+		Token:               "",
+		EnabledIssueByLabel: false,
 	}
 
 	// I18n settings