Cache feature flag names in Redis for a minute

We saw on GitLab.com, the SQL query, `SELECT "features"."key" FROM
"features"` peaked at 2300 times per second.

We can quiet this down a bit by caching it in Redis for a minute.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/63435

changelogs/unreleased/sh-cache-feature-flag-names.yml

@@ -0,0 +1,5 @@
+---
+title: Cache feature flag names in Redis for a minute
+merge_request: 29816
+author:
+type: performance

lib/feature.rb

@@ -30,7 +30,12 @@ class Feature
     end
 
     def persisted_names
-      Gitlab::SafeRequestStore[:flipper_persisted_names] ||= FlipperFeature.feature_names
+      Gitlab::SafeRequestStore[:flipper_persisted_names] ||=
+        begin
+          # We saw on GitLab.com, this database request was called 2300
+          # times/s. Let's cache it for a minute to avoid that load.
+          Rails.cache.fetch('flipper:persisted_names', expires_in: 1.minute) { FlipperFeature.feature_names }
+        end
     end
 
     def persisted?(feature)

spec/lib/feature_spec.rb

@@ -31,7 +31,8 @@ describe Feature do
       expect(described_class.persisted_names).to be_empty
     end
 
-    it 'caches the feature names when request store is active', :request_store do
+    it 'caches the feature names when request store is active',
+       :request_store, :use_clean_rails_memory_store_caching do
       Feature::FlipperFeature.create!(key: 'foo')
 
       expect(Feature::FlipperFeature)
@@ -39,6 +40,12 @@ describe Feature do
         .once
         .and_call_original
 
+      expect(Rails.cache)
+        .to receive(:fetch)
+        .once
+        .with('flipper:persisted_names', expires_in: 1.minute)
+        .and_call_original
+
       2.times do
         expect(described_class.persisted_names).to eq(%w[foo])
       end