Cache the loading hints partial

Caches the loading_hints partial on a short TTL.

app/views/layouts/_loading_hints.html.haml

-- if ActionController::Base.asset_host
-  %link{ rel: 'dns-prefetch', href: ActionController::Base.asset_host }
-  %link{ rel: 'preconnect', href: ActionController::Base.asset_host, crossorigin: '' }
-- if user_application_theme == 'gl-dark'
-  %link{ { rel: 'preload', href: stylesheet_url('application_dark'), as: 'style' }, ActionController::Base.asset_host ? { crossorigin: 'anonymous' } : {} }
-- else
-  %link{ { rel: 'preload', href: stylesheet_url('application'), as: 'style' }, ActionController::Base.asset_host ? { crossorigin: 'anonymous' } : {} }
-%link{ { rel: 'preload', href: stylesheet_url("highlight/themes/#{user_color_scheme}"), as: 'style' }, ActionController::Base.asset_host ? { crossorigin: 'anonymous' } : {} }
-- if Gitlab::CurrentSettings.snowplow_enabled? && Gitlab::CurrentSettings.snowplow_collector_hostname
-  %link{ rel: 'preconnect', href: Gitlab::CurrentSettings.snowplow_collector_hostname, crossorigin: '' }
+= cache_if(Feature.enabled?(:cached_loading_hints, current_user), [ActionController::Base.asset_host, user_application_theme, user_color_scheme], expires_in: 1.minute) do
+  - if ActionController::Base.asset_host
+    %link{ rel: 'dns-prefetch', href: ActionController::Base.asset_host }
+    %link{ rel: 'preconnect', href: ActionController::Base.asset_host, crossorigin: '' }
+  - if user_application_theme == 'gl-dark'
+    %link{ { rel: 'preload', href: stylesheet_url('application_dark'), as: 'style' }, ActionController::Base.asset_host ? { crossorigin: 'anonymous' } : {} }
+  - else
+    %link{ { rel: 'preload', href: stylesheet_url('application'), as: 'style' }, ActionController::Base.asset_host ? { crossorigin: 'anonymous' } : {} }
+  %link{ { rel: 'preload', href: stylesheet_url("highlight/themes/#{user_color_scheme}"), as: 'style' }, ActionController::Base.asset_host ? { crossorigin: 'anonymous' } : {} }
+  - if Gitlab::CurrentSettings.snowplow_enabled? && Gitlab::CurrentSettings.snowplow_collector_hostname
+    %link{ rel: 'preconnect', href: Gitlab::CurrentSettings.snowplow_collector_hostname, crossorigin: '' }

config/feature_flags/development/cached_loading_hints.yml

+---
+name: cached_loading_hints
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/61609
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/330802
+milestone: '13.12'
+type: development
+group: group::source code
+default_enabled: false