Fix tracking of issue label change events

We cannot use an after_save hook here because ChangeLabelsService uses
bulk_insert. This was working unexpectedly because after_save was being
triggered when the markdown cache was being updated.

app/models/resource_label_event.rb

@@ -15,7 +15,6 @@ class ResourceLabelEvent < ResourceEvent
   validate :exactly_one_issuable
 
   after_save :expire_etag_cache
-  after_save :usage_metrics
   after_destroy :expire_etag_cache
 
   enum action: {
@@ -114,16 +113,6 @@ class ResourceLabelEvent < ResourceEvent
   def discussion_id_key
     [self.class.name, created_at, user_id]
   end
-
-  def for_issue?
-    issue_id.present?
-  end
-
-  def usage_metrics
-    return unless for_issue?
-
-    Gitlab::UsageDataCounters::IssueActivityUniqueCounter.track_issue_label_changed_action(author: user)
-  end
 end
 
 ResourceLabelEvent.prepend_if_ee('EE::ResourceLabelEvent')

app/services/resource_events/change_labels_service.rb

@@ -24,6 +24,8 @@ module ResourceEvents
 
       Gitlab::Database.bulk_insert(ResourceLabelEvent.table_name, labels) # rubocop:disable Gitlab/BulkInsert
       resource.expire_note_etag_cache
+
+      Gitlab::UsageDataCounters::IssueActivityUniqueCounter.track_issue_label_changed_action(author: user) if resource.is_a?(Issue)
     end
 
     private

spec/models/resource_label_event_spec.rb

@@ -51,14 +51,6 @@ RSpec.describe ResourceLabelEvent, type: :model do
   end
 
   context 'callbacks' do
-    describe '#usage_metrics' do
-      it 'tracks changed labels' do
-        expect(Gitlab::UsageDataCounters::IssueActivityUniqueCounter).to receive(:track_issue_label_changed_action)
-
-        subject.save!
-      end
-    end
-
     describe '#expire_etag_cache' do
       def expect_expiration(issue)
         expect_next_instance_of(Gitlab::EtagCaching::Store) do |instance|

spec/services/resource_events/change_labels_service_spec.rb

@@ -10,7 +10,7 @@ RSpec.describe ResourceEvents::ChangeLabelsService do
   describe '.change_labels' do
     subject { described_class.new(resource, author).execute(added_labels: added, removed_labels: removed) }
 
-    let(:labels) { create_list(:label, 2, project: project) }
+    let_it_be(:labels) { create_list(:label, 2, project: project) }
 
     def expect_label_event(event, label, action)
       expect(event.user).to eq(author)
@@ -57,5 +57,28 @@ RSpec.describe ResourceEvents::ChangeLabelsService do
         expect { subject }.to change { resource.resource_label_events.count }.from(0).to(2)
       end
     end
+
+    describe 'usage data' do
+      let(:added)   { [labels[0]] }
+      let(:removed) { [labels[1]] }
+
+      context 'when resource is an issue' do
+        it 'tracks changed labels' do
+          expect(Gitlab::UsageDataCounters::IssueActivityUniqueCounter).to receive(:track_issue_label_changed_action)
+
+          subject
+        end
+      end
+
+      context 'when resource is a merge request' do
+        let(:resource) { create(:merge_request, source_project: project) }
+
+        it 'does not track changed labels' do
+          expect(Gitlab::UsageDataCounters::IssueActivityUniqueCounter).not_to receive(:track_issue_label_changed_action)
+
+          subject
+        end
+      end
+    end
   end
 end