Add 2 endpoints to return issue and MR count

- These return counts for issues/MRs visible to
the current user, for the given group

ee/changelogs/unreleased/ag-add-feature-flsg-for-group-activity.yml

+---
+title: Add API endpoints for Group Activity Analytics
+merge_request: 25606
+author:
+type: added

ee/lib/api/analytics/group_activity_analytics.rb

+# frozen_string_literal: true
+
+module API
+  module Analytics
+    class GroupActivityAnalytics < Grape::API
+      DESCRIPTION_DETAIL =
+        'This feature is gated by the `:group_activity_analytics`'\
+        ' feature flag, introduced in GitLab 12.9.'
+
+      before do
+        authenticate!
+        not_found! unless Feature.enabled?(:group_activity_analytics)
+      end
+
+      helpers do
+        def group
+          @group ||= find_group!(params[:group_path])
+        end
+
+        def calculator
+          @calculator ||=
+            ::Analytics::GroupActivityCalculator.new(group, current_user)
+        end
+      end
+
+      resource :analytics do
+        resource :group_activity do
+          desc 'Get count of recently created issues for group' do
+            detail DESCRIPTION_DETAIL
+            success EE::API::Entities::Analytics::GroupActivity::IssuesCount
+          end
+
+          params do
+            requires :group_path, type: String, desc: 'Group Path'
+          end
+
+          get 'issues_count' do
+            authorize! :read_group_activity_analytics, group
+            present(
+              calculator,
+              with: EE::API::Entities::Analytics::GroupActivity::IssuesCount
+            )
+          end
+
+          desc 'Get count of recently created merge requests for group' do
+            detail DESCRIPTION_DETAIL
+            success EE::API::Entities::Analytics::GroupActivity::MergeRequestsCount
+          end
+
+          params do
+            requires :group_path, type: String, desc: 'Group Path'
+          end
+
+          get 'merge_requests_count' do
+            authorize! :read_group_activity_analytics, group
+            present(
+              calculator,
+              with: EE::API::Entities::Analytics::GroupActivity::MergeRequestsCount
+            )
+          end
+        end
+      end
+    end
+  end
+end

ee/lib/ee/api/api.rb

@@ -52,6 +52,7 @@ module EE
         mount ::API::Dependencies
         mount ::API::VisualReviewDiscussions
         mount ::API::Analytics::CodeReviewAnalytics
+        mount ::API::Analytics::GroupActivityAnalytics
         mount ::API::ProtectedEnvironments
 
         version 'v3', using: :path do

ee/lib/ee/api/entities.rb

@@ -512,6 +512,16 @@ module EE
             # rubocop: enable CodeReuse/ActiveRecord
           end
         end
+
+        module GroupActivity
+          class IssuesCount < Grape::Entity
+            expose :issues_count
+          end
+
+          class MergeRequestsCount < Grape::Entity
+            expose :merge_requests_count
+          end
+        end
       end
 
       module UserDetailsWithAdmin

ee/spec/lib/ee/api/entities/analytics/group_activity_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe EE::API::Entities::Analytics::GroupActivity do
+  let(:count) { 10 }
+
+  shared_examples 'exposes count' do |count_type, entity|
+    let(:data) { { count_type.to_sym => count } }
+
+    subject(:entity_representation) { entity.new(data).as_json }
+
+    it "exposes #{count_type}" do
+      expect(entity_representation).to include({ count_type.to_sym => count })
+    end
+  end
+
+  context 'issues count' do
+    it_behaves_like 'exposes count', 'issues_count', EE::API::Entities::Analytics::GroupActivity::IssuesCount
+  end
+
+  context 'merge requests count' do
+    it_behaves_like 'exposes count', 'merge_requests_count', EE::API::Entities::Analytics::GroupActivity::MergeRequestsCount
+  end
+end

ee/spec/requests/api/analytics/group_activity_analytics_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe API::Analytics::GroupActivityAnalytics do
+  let_it_be(:group) { create(:group, :private) }
+
+  let_it_be(:reporter) do
+    create(:user).tap { |u| group.add_reporter(u) }
+  end
+
+  let_it_be(:anonymous_user) { create(:user) }
+
+  shared_examples 'GET group_activity' do |activity|
+    let(:feature_enabled) { true }
+    let(:params) { { group_path: group.full_path } }
+    let(:current_user) { reporter }
+    let(:request) do
+      get api("/analytics/group_activity/#{activity}_count", current_user), params: params
+    end
+
+    before do
+      stub_licensed_features(group_activity_analytics: feature_enabled)
+      request
+    end
+
+    it 'is successful' do
+      expect(response).to have_gitlab_http_status(:ok)
+    end
+
+    it 'is returns a count' do
+      expect(response.parsed_body).to eq({ "#{activity}_count" => 0 })
+    end
+
+    context 'when feature is not available in plan' do
+      let(:feature_enabled) { false }
+
+      it 'is returns `forbidden`' do
+        expect(response).to have_gitlab_http_status(:forbidden)
+      end
+    end
+
+    context 'when `group_path` is not specified' do
+      let(:params) { }
+
+      it 'is returns `bad_request`' do
+        expect(response).to have_gitlab_http_status(:bad_request)
+      end
+    end
+
+    context 'when user has no authorization to view a private group' do
+      let(:current_user) { anonymous_user }
+
+      it 'is returns `not_found`' do
+        expect(response).to have_gitlab_http_status(:not_found)
+      end
+    end
+  end
+
+  context 'GET /group_activity/issues_count' do
+    it_behaves_like 'GET group_activity', 'issues'
+  end
+
+  context 'GET /group_activity/merge_requests_count' do
+    it_behaves_like 'GET group_activity', 'merge_requests'
+  end
+end