Add connection fallback to ActiveRecordProxy

As observed in https://gitlab.com/gitlab-org/gitlab/-/issues/341584,
prepending ActiveRecordProxy before setting up the proxy may result in
database queries being run that expect a connection. To work around
this, ActiveRecordProxy now falls back to retrieving a regular
connection when a proxy isn't available yet.

Changelog: fixed

lib/gitlab/database/load_balancing/active_record_proxy.rb

@@ -7,7 +7,7 @@ module Gitlab
       # "connection" method.
       module ActiveRecordProxy
         def connection
-          ::Gitlab::Database::LoadBalancing.proxy
+          ::Gitlab::Database::LoadBalancing.proxy || super
         end
       end
     end

spec/lib/gitlab/database/load_balancing/active_record_proxy_spec.rb

@@ -16,5 +16,12 @@ RSpec.describe Gitlab::Database::LoadBalancing::ActiveRecordProxy do
 
       expect(dummy.new.connection).to eq(proxy)
     end
+
+    it 'returns a connection when no proxy is present' do
+      allow(Gitlab::Database::LoadBalancing).to receive(:proxy).and_return(nil)
+
+      expect(ActiveRecord::Base.connection)
+        .to eq(ActiveRecord::Base.retrieve_connection)
+    end
   end
 end