Create add_severity_system_note feature flag

Hide adding a severity system note behind feature flag

app/services/system_notes/incident_service.rb

@@ -10,6 +10,8 @@ module SystemNotes
     #
     # Returns the created Note object
     def change_incident_severity
+      return unless Feature.enabled?(:add_severity_system_note, noteable.project)
+
       severity = noteable.severity
 
       if severity_label = IssuableSeverity::SEVERITY_LABELS[severity.to_sym]

config/feature_flags/development/add_severity_system_note.yml

+---
+name: add_severity_system_note
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/42358
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/251110
+group: group::health
+type: development
+default_enabled: false

spec/services/system_notes/incident_service_spec.rb

@@ -11,45 +11,68 @@ RSpec.describe ::SystemNotes::IncidentService do
   describe '#change_incident_severity' do
     subject(:change_severity) { described_class.new(noteable: noteable, project: project, author: author).change_incident_severity }
 
-    it_behaves_like 'a system note' do
-      let(:action) { 'severity' }
+    before do
+      allow(Gitlab::AppLogger).to receive(:error).and_call_original
     end
 
-    IssuableSeverity.severities.keys.each do |severity|
-      context "with #{severity} severity" do
+    context 'with add_severity_system_note feature flag enabled' do
+      before do
+        stub_feature_flags(add_severity_system_note: project)
+      end
+
+      it_behaves_like 'a system note' do
+        let(:action) { 'severity' }
+      end
+
+      IssuableSeverity.severities.keys.each do |severity|
+        context "with #{severity} severity" do
+          before do
+            issuable_severity.update!(severity: severity)
+          end
+
+          it 'has the appropriate message' do
+            severity_label = IssuableSeverity::SEVERITY_LABELS.fetch(severity.to_sym)
+
+            expect(change_severity.note).to eq("changed the severity to **#{severity_label}**")
+          end
+        end
+      end
+
+      context 'when severity is invalid' do
+        let(:invalid_severity) { 'invalid-severity' }
+
         before do
-          issuable_severity.update!(severity: severity)
+          allow(noteable).to receive(:severity).and_return(invalid_severity)
         end
 
-        it 'has the appropriate message' do
-          severity_label = IssuableSeverity::SEVERITY_LABELS.fetch(severity.to_sym)
+        it 'does not create system note' do
+          expect { change_severity }.not_to change { noteable.notes.count }
+        end
 
-          expect(change_severity.note).to eq("changed the severity to **#{severity_label}**")
+        it 'writes error to logs' do
+          change_severity
+
+          expect(Gitlab::AppLogger).to have_received(:error).with(
+            message: 'Cannot create a system note for severity change',
+            noteable_class: noteable.class.to_s,
+            noteable_id: noteable.id,
+            severity: invalid_severity
+          )
         end
       end
     end
 
-    context 'when severity is invalid' do
-      let(:invalid_severity) { 'invalid-severity' }
-
+    context 'with add_severity_system_note feature flag disabled' do
       before do
-        allow(noteable).to receive(:severity).and_return(invalid_severity)
-        allow(Gitlab::AppLogger).to receive(:error).and_call_original
+        stub_feature_flags(add_severity_system_note: false)
       end
 
       it 'does not create system note' do
         expect { change_severity }.not_to change { noteable.notes.count }
       end
 
-      it 'writes error to logs' do
-        change_severity
-
-        expect(Gitlab::AppLogger).to have_received(:error).with(
-          message: 'Cannot create a system note for severity change',
-          noteable_class: noteable.class.to_s,
-          noteable_id: noteable.id,
-          severity: invalid_severity
-        )
+      it 'does not write error to logs' do
+        expect(Gitlab::AppLogger).not_to have_received(:error)
       end
     end
   end