Create optional_approvals_count method

Issue: gitlab.com/gitlab-org/gitlab/-/issues/209786
Merge Request: gitlab.com/gitlab-org/gitlab/-/merge_requests/32886

ee/app/models/approval_state.rb

@@ -167,6 +167,12 @@ class ApprovalState
     end
   end
 
+  def optional_approvals_count
+    strong_memoize(:optional_approvals_count) do
+      wrapped_approval_rules.sum { |rule| [0, rule.approved_approvers.count - rule.approvals_required].max }
+    end
+  end
+
   private
 
   attr_reader :target_branch

ee/app/models/approval_wrapped_rule.rb

@@ -36,7 +36,7 @@ class ApprovalWrappedRule
     filter_approvers(@approval_rule.approvers)
   end
 
-  # @return [Array<User>] all approvers related to this rule
+  # @return [Array<User>] of users who have approved the merge request
   #
   # This is dynamically calculated unless it is persisted as `approved_approvers`.
   #

ee/spec/models/approval_state_spec.rb

@@ -1637,4 +1637,51 @@ describe ApprovalState do
       end
     end
   end
+
+  describe '#optional_approvals_count' do
+    context 'when there are no optional approvals' do
+      let(:rule) { create_rule(approvals_required: 5) }
+
+      before do
+        approve_rules([rule])
+      end
+
+      it 'returns 0' do
+        expect(subject.optional_approvals_count).to eq(0)
+      end
+    end
+
+    context 'when there are no approvals' do
+      let(:rule) { create_rule(approvals_required: 5) }
+
+      it 'returns 0' do
+        expect(subject.optional_approvals_count).to eq(0)
+      end
+    end
+
+    context 'when there are only optional approvals' do
+      let(:rule) { create_rule(approvals_required: 0) }
+
+      before do
+        create(:approval, merge_request: merge_request, user: rule.users.first)
+      end
+
+      it 'returns the number of approvals' do
+        expect(subject.optional_approvals_count).to eq(1)
+      end
+    end
+
+    context 'when there are both required and optional approvals' do
+      let(:rule) { create_rule(approvals_required: 1, rule_type: :any_approver, users: [approver1, approver2]) }
+
+      before do
+        create(:approval, merge_request: merge_request, user: rule.users.first)
+        create(:approval, merge_request: merge_request, user: rule.users.second)
+      end
+
+      it 'returns the number of optional approvals' do
+        expect(subject.optional_approvals_count).to eq(1)
+      end
+    end
+  end
 end