Add a calculator for Group Activity analytics

This class would run queries for all stats we
want to show, as authorized for the current user.

ee/lib/analytics/group_activity_calculator.rb

+# frozen_string_literal: true
+
+module Analytics
+  class GroupActivityCalculator
+    DURATION = 90.days
+
+    def initialize(group, current_user)
+      @group = group
+      @current_user = current_user
+    end
+
+    def issues_count
+      @issues_count ||=
+        IssuesFinder.new(@current_user, params).execute.count
+    end
+
+    def merge_requests_count
+      @merge_requests_count ||=
+        MergeRequestsFinder.new(@current_user, params).execute.count
+    end
+
+    private
+
+    def params
+      { group_id: @group.id,
+        state: 'all',
+        created_after: DURATION.ago,
+        include_subgroups: true,
+        attempt_group_search_optimizations: true,
+        attempt_project_search_optimizations: true }
+    end
+  end
+end

ee/spec/lib/analytics/group_activity_calculator_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Analytics::GroupActivityCalculator do
+  subject { described_class.new(group, current_user) }
+
+  set(:group) { create(:group) }
+  set(:current_user) { create(:user) }
+  set(:subgroup) { create(:group, parent: group) }
+  set(:project) { create(:project, group: subgroup) }
+  set(:secret_subgroup) { create(:group, parent: group) }
+  set(:secret_project) { create(:project, group: secret_subgroup) }
+
+  before do
+    subgroup.add_developer(current_user)
+    project.add_developer(current_user)
+  end
+
+  context 'with issues' do
+    set(:recent_issue) { create(:issue, project: project) }
+    set(:old_issue) do
+      create(:issue,
+        project: project,
+        created_at: 100.days.ago)
+    end
+
+    it 'only returns the count of recent issues' do
+      expect(subject.issues_count).to eq 1
+    end
+
+    context 'when user does not have access to some issues' do
+      let(:secret_issue) { create(:issue, project: secret_project) }
+
+      it 'does not include those issues' do
+        expect { secret_issue }.not_to change { subject.issues_count}
+      end
+    end
+  end
+
+  context 'with merge requests' do
+    let!(:recent_mr) do
+      create(:merge_request,
+        source_project: project,
+        source_branch: "my-personal-branch-1")
+    end
+
+    let!(:old_mr) do
+      create(:merge_request,
+        source_project: project,
+        source_branch: "my-personal-branch-2",
+        created_at: 100.days.ago)
+    end
+
+    it 'only returns the count of recent MRs' do
+      expect(subject.merge_requests_count).to eq 1
+    end
+
+    context 'when user does not have access to some MRs' do
+      let(:secret_mr) { create(:merge_request, source_project: secret_project) }
+
+      it 'does not include those MRs' do
+        expect { secret_mr }.not_to change { subject.merge_requests_count}
+      end
+    end
+  end
+end