Add feature flag

I'm adding a feature flag that will allow us to disable this feature in
case we detect a performance issue.

ee/app/models/gitlab_subscription.rb

@@ -94,6 +94,7 @@ class GitlabSubscription < ApplicationRecord
   # We need to show seats in use for free or trial subscriptions
   # in order to make it easy for customers to get this information.
   def seats_in_use
+    return super unless Feature.enabled?(:seats_in_use_for_free_or_trial, type: :ops, default_enabled: true)
     return super if has_a_paid_hosted_plan? || !hosted?
 
     seats_in_use_now

ee/config/feature_flags/ops/seats-in-use-for-free-or-trial.yml

+---
+name: seats-in-use-for-free-or-trial
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/44973
+rollout_issue_url: 
+type: ops
+group: group::provision
+default_enabled: true

ee/spec/models/gitlab_subscription_spec.rb

@@ -211,6 +211,18 @@ RSpec.describe GitlabSubscription do
       create(:gitlab_subscription, namespace: group, trial: trial, hosted_plan: hosted_plan, seats_in_use: seats_in_use)
     end
 
+    shared_examples 'a disabled feature' do
+      context 'when feature flag is disabled' do
+        before do
+          stub_feature_flags(seats_in_use_for_free_or_trial: false)
+        end
+
+        it 'returns the previously calculated seats in use' do
+          expect(subject).to eq(5)
+        end
+      end
+    end
+
     subject { gitlab_subscription.seats_in_use }
 
     context 'with a paid hosted plan' do
@@ -236,6 +248,8 @@ RSpec.describe GitlabSubscription do
       it 'returns the current seats in use' do
         expect(subject).to eq(1)
       end
+
+      it_behaves_like 'a disabled feature'
     end
 
     context 'with a free plan' do
@@ -244,6 +258,8 @@ RSpec.describe GitlabSubscription do
       it 'returns the current seats in use' do
         expect(subject).to eq(1)
       end
+
+      it_behaves_like 'a disabled feature'
     end
 
     context 'with a self hosted plan' do