Set `first_mentioned_in_commit_at` on issue closes

This change sets the `first_mentioned_in_commit_at` attribute of the
issue metrics record to the first commit in the MR. The change is
behind feature flag: store_first_mentioned_in_commit_on_issue_close

app/services/issues/close_service.rb

@@ -36,6 +36,8 @@ module Issues
         execute_hooks(issue, 'close')
         invalidate_cache_counts(issue, users: issue.assignees)
         issue.update_project_counter_caches
+
+        store_first_mentioned_in_commit_at(issue, closed_via) if closed_via.is_a?(MergeRequest)
       end
 
       issue
@@ -46,5 +48,17 @@ module Issues
     def create_note(issue, current_commit)
       SystemNoteService.change_status(issue, issue.project, current_user, issue.state, current_commit)
     end
+
+    def store_first_mentioned_in_commit_at(issue, merge_request)
+      return unless Feature.enabled?(:store_first_mentioned_in_commit_on_issue_close, issue.project)
+
+      metrics = issue.metrics
+      return if metrics.nil? || metrics.first_mentioned_in_commit_at
+
+      first_commit_timestamp = merge_request.commits(limit: 1).first&.date
+      return unless first_commit_timestamp
+
+      metrics.update!(first_mentioned_in_commit_at: first_commit_timestamp)
+    end
   end
 end

spec/services/issues/close_service_spec.rb

@@ -98,6 +98,53 @@ describe Issues::CloseService do
           expect(body_text).not_to include(closing_merge_request.to_reference)
         end
       end
+
+      context 'updating `metrics.first_mentioned_in_commit_at`' do
+        subject { described_class.new(project, user).close_issue(issue, closed_via: closing_merge_request) }
+
+        context 'when `metrics.first_mentioned_in_commit_at` is not set' do
+          it 'uses the first commit timestamp' do
+            expected = closing_merge_request.commits.first.date
+
+            subject
+
+            expect(issue.metrics.first_mentioned_in_commit_at).to eq(expected)
+          end
+        end
+
+        context 'when `metrics.first_mentioned_in_commit_at` is already set' do
+          before do
+            issue.metrics.update!(first_mentioned_in_commit_at: Time.now)
+          end
+
+          it 'does not update the metrics' do
+            expect { subject }.not_to change { issue.metrics.first_mentioned_in_commit_at }
+          end
+        end
+
+        context 'when merge request has no commits' do
+          let(:closing_merge_request) { create(:merge_request, :without_diffs, source_project: project) }
+
+          it 'does not update the metrics' do
+            subject
+
+            expect(issue.metrics.first_mentioned_in_commit_at).to be_nil
+          end
+        end
+
+        context 'when `store_first_mentioned_in_commit_on_issue_close` feature flag is off' do
+          before do
+            stub_feature_flags(store_first_mentioned_in_commit_on_issue_close: { enabled: false, thing: issue.project })
+          end
+
+          it 'does not update the metrics' do
+            subject
+
+            expect(described_class).not_to receive(:store_first_mentioned_in_commit_at)
+            expect(issue.metrics.first_mentioned_in_commit_at).to be_nil
+          end
+        end
+      end
     end
 
     context "closed by a commit", :sidekiq_might_not_need_inline do

spec/services/merge_requests/merge_service_spec.rb

@@ -118,7 +118,7 @@ describe MergeRequests::MergeService do
 
       it 'closes GitLab issue tracker issues' do
         issue  = create :issue, project: project
-        commit = double('commit', safe_message: "Fixes #{issue.to_reference}")
+        commit = double('commit', safe_message: "Fixes #{issue.to_reference}", date: Time.now)
         allow(merge_request).to receive(:commits).and_return([commit])
         merge_request.cache_merge_request_closes_issues!