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

@@ -11,6 +11,15 @@ 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 }
 
+    before do
+      allow(Gitlab::AppLogger).to receive(:error).and_call_original
+    end
+
+    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
@@ -34,7 +43,6 @@ RSpec.describe ::SystemNotes::IncidentService do
 
         before do
           allow(noteable).to receive(:severity).and_return(invalid_severity)
-        allow(Gitlab::AppLogger).to receive(:error).and_call_original
         end
 
         it 'does not create system note' do
@@ -53,4 +61,19 @@ RSpec.describe ::SystemNotes::IncidentService do
         end
       end
     end
+
+    context 'with add_severity_system_note feature flag disabled' do
+      before do
+        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 'does not write error to logs' do
+        expect(Gitlab::AppLogger).not_to have_received(:error)
+      end
+    end
+  end
 end