Cache Flipper persisted names directly to local memory storage

Now that application settings are no longer dominating network traffic,
we see that the Feature#persisted_names is using a significant amount of
CPU and network bandwidth for Redis. Move this cache into the
thread-local memory storage to reduce Redis overhead.

changelogs/unreleased/sh-cache-flipper-names-memory-cache.yml

+---
+title: Cache Flipper persisted names directly to local memory storage
+merge_request: 30265
+author:
+type: performance

lib/feature.rb

@@ -34,7 +34,9 @@ class Feature
         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 }
+          Gitlab::ThreadMemoryCache.cache_backend.fetch('flipper:persisted_names', expires_in: 1.minute) do
+            FlipperFeature.feature_names
+          end
         end
     end
 

spec/lib/feature_spec.rb

@@ -40,7 +40,7 @@ describe Feature do
         .once
         .and_call_original
 
-      expect(Rails.cache)
+      expect(Gitlab::ThreadMemoryCache.cache_backend)
         .to receive(:fetch)
         .once
         .with('flipper:persisted_names', expires_in: 1.minute)