Mark feature flags in Gitlab/MarkUsedFeatureFlags only once

Mark dynamic feature flags and feature flags for usage data counters
only once per whole rubocop run instead of once per file.

This reduces the overall runtime of this cop drastically:

    time bundle exec rubocop -C false -f q --only Gitlab/MarkUsedFeatureFlags

* Before: ~8m53.589s
* After: ~3m56.036s

rubocop/cop/gitlab/mark_used_feature_flags.rb

@@ -47,10 +47,21 @@ class MarkUsedFeatureFlags < RuboCop::Cop::Cop
           :usage_data_static_site_editor_merge_requests # https://gitlab.com/gitlab-org/gitlab/-/issues/284083
         ].freeze
 
+        class << self
+          # We track feature flags in `on_new_investigation` only once per
+          # rubocop whole run instead once per file.
+          attr_accessor :feature_flags_already_tracked
+        end
+
         # Called before all on_... have been called
         # When refining this method, always call `super`
         def on_new_investigation
           super
+
+          return if self.class.feature_flags_already_tracked
+
+          self.class.feature_flags_already_tracked = true
+
           track_dynamic_feature_flags!
           track_usage_data_counters_known_events!
         end

spec/rubocop/cop/gitlab/mark_used_feature_flags_spec.rb

@@ -16,6 +16,7 @@
     stub_const("#{described_class}::DYNAMIC_FEATURE_FLAGS", [])
     allow(cop).to receive(:defined_feature_flags).and_return(defined_feature_flags)
     allow(cop).to receive(:usage_data_counters_known_event_feature_flags).and_return([])
+    described_class.feature_flags_already_tracked = false
   end
 
   def feature_flag_path(feature_flag_name)