Add circuit breaker for gitlab experimentation

- a quick way to kill all experiments on SaaS

Changelog: added

config/feature_flags/ops/gitlab_experiment.yml

+---
+name: gitlab_experiment
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/81834
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/353921
+milestone: '14.9'
+type: ops
+group: group::conversion
+default_enabled: true

lib/gitlab/experiment/rollout/feature.rb

@@ -12,10 +12,11 @@ module Gitlab
         # - not have rolled out the feature flag at all (no percent of actors,
         #   no inclusions, etc.)
         def enabled?
-          return false if ::Feature::Definition.get(feature_flag_name).nil?
+          return false unless feature_flag_defined?
           return false unless Gitlab.dev_env_or_com?
+          return false unless ::Feature.enabled?(:gitlab_experiment, type: :ops, default_enabled: :yaml)
 
-          ::Feature.get(feature_flag_name).state != :off # rubocop:disable Gitlab/AvoidFeatureGet
+          feature_flag_instance.state != :off
         end
 
         # For assignment we first check to see if our feature flag is enabled
@@ -58,6 +59,14 @@ module Gitlab
 
         private
 
+        def feature_flag_instance
+          ::Feature.get(feature_flag_name) # rubocop:disable Gitlab/AvoidFeatureGet
+        end
+
+        def feature_flag_defined?
+          ::Feature::Definition.get(feature_flag_name).present?
+        end
+
         def feature_flag_name
           experiment.name.tr('/', '_')
         end

spec/lib/gitlab/experiment/rollout/feature_spec.rb

@@ -9,9 +9,10 @@ RSpec.describe Gitlab::Experiment::Rollout::Feature, :experiment do
 
   describe "#enabled?" do
     before do
-      allow(Feature::Definition).to receive(:get).and_return('_instance_')
+      stub_feature_flags(gitlab_experiment: true)
+      allow(subject).to receive(:feature_flag_defined?).and_return(true)
       allow(Gitlab).to receive(:dev_env_or_com?).and_return(true)
-      allow(Feature).to receive(:get).and_return(double(state: :on))
+      allow(subject).to receive(:feature_flag_instance).and_return(double(state: :on))
     end
 
     it "is enabled when all criteria are met" do
@@ -19,7 +20,7 @@ RSpec.describe Gitlab::Experiment::Rollout::Feature, :experiment do
     end
 
     it "isn't enabled if the feature definition doesn't exist" do
-      expect(Feature::Definition).to receive(:get).with('namespaced_stub').and_return(nil)
+      expect(subject).to receive(:feature_flag_defined?).and_return(false)
 
       expect(subject).not_to be_enabled
     end
@@ -31,7 +32,13 @@ RSpec.describe Gitlab::Experiment::Rollout::Feature, :experiment do
     end
 
     it "isn't enabled if the feature flag state is :off" do
-      expect(Feature).to receive(:get).with('namespaced_stub').and_return(double(state: :off))
+      expect(subject).to receive(:feature_flag_instance).and_return(double(state: :off))
+
+      expect(subject).not_to be_enabled
+    end
+
+    it "isn't enabled if the gitlab_experiment feature flag is false" do
+      stub_feature_flags(gitlab_experiment: false)
 
       expect(subject).not_to be_enabled
     end