Track the number of users who are assigned and get requested for reviews

Issue: gitlab.com/gitlab-org/gitlab/-/issues/299087
MR: gitlab.com/gitlab-org/gitlab/-/merge_requests/52194

app/services/merge_requests/update_service.rb

@@ -109,6 +109,9 @@ module MergeRequests
       create_assignee_note(merge_request, old_assignees)
       notification_service.async.reassigned_merge_request(merge_request, current_user, old_assignees)
       todo_service.reassigned_assignable(merge_request, current_user, old_assignees)
+
+      new_assignees = merge_request.assignees - old_assignees
+      merge_request_activity_counter.track_users_assigned_to_mr(users: new_assignees)
     end
 
     def handle_reviewers_change(merge_request, old_reviewers)
@@ -117,6 +120,9 @@ module MergeRequests
       notification_service.async.changed_reviewer_of_merge_request(merge_request, current_user, old_reviewers)
       todo_service.reassigned_reviewable(merge_request, current_user, old_reviewers)
       invalidate_cache_counts(merge_request, users: affected_reviewers.compact)
+
+      new_reviewers = merge_request.reviewers - old_reviewers
+      merge_request_activity_counter.track_users_review_requested(users: new_reviewers)
     end
 
     def create_branch_change_note(issuable, branch_type, old_branch, new_branch)

changelogs/unreleased/299087-track-assignees-and-reviewers-for-merge-requests.yml

+---
+title: Track the number of users who are assigned and get requested for reviews
+merge_request: 52194
+author:
+type: other

config/feature_flags/development/usage_data_i_code_review_user_assigned.yml

+---
+name: usage_data_i_code_review_user_assigned
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/52194
+rollout_issue_url: 
+milestone: '13.9'
+type: development
+group: group::code review
+default_enabled: true

config/feature_flags/development/usage_data_i_code_review_user_review_requested.yml

+---
+name: usage_data_i_code_review_user_review_requested
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/52194
+rollout_issue_url: 
+milestone: '13.9'
+type: development
+group: group::code review
+default_enabled: true

ee/spec/services/ee/merge_requests/update_service_spec.rb

@@ -330,5 +330,25 @@ RSpec.describe MergeRequests::UpdateService, :mailer do
 
       update_merge_request(title: 'Title')
     end
+
+    context 'updating assignee_ids' do
+      it 'updates the tracking when user ids are valid' do
+        expect(Gitlab::UsageDataCounters::MergeRequestActivityUniqueCounter)
+          .to receive(:track_users_assigned_to_mr)
+          .with(users: [user, user2])
+
+        update_merge_request(assignee_ids: [user.id, user2.id])
+      end
+    end
+
+    context 'updating reviewers_ids' do
+      it 'updates the tracking when user ids are valid' do
+        expect(Gitlab::UsageDataCounters::MergeRequestActivityUniqueCounter)
+          .to receive(:track_users_review_requested)
+          .with(users: [user, user2])
+
+        update_merge_request(reviewer_ids: [user.id, user2.id])
+      end
+    end
   end
 end

lib/gitlab/usage_data_counters/known_events/common.yml

@@ -541,6 +541,16 @@
   category: code_review
   aggregation: weekly
   feature_flag: usage_data_i_code_review_user_apply_suggestion
+- name: i_code_review_user_assigned
+  redis_slot: code_review
+  category: code_review
+  aggregation: weekly
+  feature_flag: usage_data_i_code_review_user_assigned
+- name: i_code_review_user_review_requested
+  redis_slot: code_review
+  category: code_review
+  aggregation: weekly
+  feature_flag: usage_data_i_code_review_user_review_requested
 # Terraform
 - name: p_terraform_state_api_unique_users
   category: terraform

lib/gitlab/usage_data_counters/merge_request_activity_unique_counter.rb

@@ -22,6 +22,8 @@ module Gitlab
       MR_APPLY_SUGGESTION_ACTION = 'i_code_review_user_apply_suggestion'
       MR_RESOLVE_THREAD_ACTION = 'i_code_review_user_resolve_thread'
       MR_UNRESOLVE_THREAD_ACTION = 'i_code_review_user_unresolve_thread'
+      MR_ASSIGNED_USERS_ACTION = 'i_code_review_user_assigned'
+      MR_REVIEW_REQUESTED_USERS_ACTION = 'i_code_review_user_review_requested'
 
       class << self
         def track_mr_diffs_action(merge_request:)
@@ -88,6 +90,14 @@ module Gitlab
           track_unique_action_by_user(MR_APPLY_SUGGESTION_ACTION, user)
         end
 
+        def track_users_assigned_to_mr(users:)
+          track_unique_action_by_users(MR_ASSIGNED_USERS_ACTION, users)
+        end
+
+        def track_users_review_requested(users:)
+          track_unique_action_by_users(MR_REVIEW_REQUESTED_USERS_ACTION, users)
+        end
+
         private
 
         def track_unique_action_by_merge_request(action, merge_request)
@@ -100,6 +110,12 @@ module Gitlab
           track_unique_action(action, user.id)
         end
 
+        def track_unique_action_by_users(action, users)
+          return if users.blank?
+
+          track_unique_action(action, users.map(&:id))
+        end
+
         def track_unique_action(action, value)
           Gitlab::UsageDataCounters::HLLRedisCounter.track_usage_event(action, value)
         end

spec/lib/gitlab/usage_data_counters/merge_request_activity_unique_counter_spec.rb

@@ -180,4 +180,20 @@ RSpec.describe Gitlab::UsageDataCounters::MergeRequestActivityUniqueCounter, :cl
       let(:action) { described_class::MR_APPLY_SUGGESTION_ACTION }
     end
   end
+
+  describe '.track_users_assigned_to_mr' do
+    subject { described_class.track_users_assigned_to_mr(users: [user]) }
+
+    it_behaves_like 'a tracked merge request unique event' do
+      let(:action) { described_class::MR_ASSIGNED_USERS_ACTION }
+    end
+  end
+
+  describe '.track_users_review_requested' do
+    subject { described_class.track_users_review_requested(users: [user]) }
+
+    it_behaves_like 'a tracked merge request unique event' do
+      let(:action) { described_class::MR_REVIEW_REQUESTED_USERS_ACTION }
+    end
+  end
 end

spec/services/merge_requests/update_service_spec.rb

@@ -143,7 +143,7 @@ RSpec.describe MergeRequests::UpdateService, :mailer do
         let(:opts) { { reviewer_ids: [user2.id] } }
 
         context 'when merge_request_reviewers feature is disabled' do
-          before(:context) do
+          before do
             stub_feature_flags(merge_request_reviewers: false)
           end
 
@@ -152,6 +152,13 @@ RSpec.describe MergeRequests::UpdateService, :mailer do
 
             expect(note).to be_nil
           end
+
+          it 'does not update the tracking' do
+            expect(Gitlab::UsageDataCounters::MergeRequestActivityUniqueCounter)
+              .not_to receive(:track_users_review_requested)
+
+            update_merge_request(reviewer_ids: [user.id])
+          end
         end
 
         context 'when merge_request_reviewers feature is enabled' do
@@ -165,6 +172,14 @@ RSpec.describe MergeRequests::UpdateService, :mailer do
             expect(note).not_to be_nil
             expect(note.note).to include "requested review from #{user2.to_reference}"
           end
+
+          it 'updates the tracking' do
+            expect(Gitlab::UsageDataCounters::MergeRequestActivityUniqueCounter)
+              .to receive(:track_users_review_requested)
+              .with(users: [user])
+
+            update_merge_request(reviewer_ids: [user.id])
+          end
         end
       end
 
@@ -794,6 +809,14 @@ RSpec.describe MergeRequests::UpdateService, :mailer do
         expect(merge_request.assignee_ids).to eq([user.id])
       end
 
+      it 'updates the tracking when user ids are valid' do
+        expect(Gitlab::UsageDataCounters::MergeRequestActivityUniqueCounter)
+          .to receive(:track_users_assigned_to_mr)
+          .with(users: [user])
+
+        update_merge_request(assignee_ids: [user.id])
+      end
+
       it 'does not update assignee_id when user cannot read issue' do
         non_member = create(:user)
         original_assignees = merge_request.assignees