Reconfigure DB load balancing connection on code reload

In development, when code is reloaded, we lose the connection class
attribute. So we need to setup a to_prepare block to reconfigure DB load
balancing.

Also add logging for when DB load balancing setup runs.

Note: whether code reloads depends on config.cache_classes,
and config.reload_classes_only_on_change. In production,
config.cache_classes is true, so no code reloading happens there.

Changelog: fixed

config/initializers/load_balancing.rb

@@ -2,6 +2,15 @@
 
 Gitlab::Application.configure do |config|
   config.middleware.use(Gitlab::Database::LoadBalancing::RackMiddleware)
+
+  # We need re-rerun the setup when code reloads in development
+  config.reloader.to_prepare do
+    if Rails.env.development? || Rails.env.test?
+      Gitlab::Database::LoadBalancing.base_models.each do |model|
+        Gitlab::Database::LoadBalancing::Setup.new(model).setup
+      end
+    end
+  end
 end
 
 Gitlab::Database::LoadBalancing.base_models.each do |model|
@@ -14,6 +23,12 @@ Gitlab::Database::LoadBalancing.base_models.each do |model|
   # information.
   Gitlab::Database::LoadBalancing::Setup.new(model).setup
 
+  Rails.application.reloader.to_prepare do
+    if Rails.env.development?
+      Gitlab::Database::LoadBalancing::Setup.new(model).setup
+    end
+  end
+
   # Database queries may be run before we fork, so we must set up the load
   # balancer as early as possible. When we do fork, we need to make sure all the
   # hosts are disconnected.

lib/gitlab/database/load_balancing/setup.rb

@@ -17,6 +17,12 @@ module Gitlab
           configure_connection
           setup_connection_proxy
           setup_service_discovery
+
+          ::Gitlab::Database::LoadBalancing::Logger.debug(
+            event: :setup,
+            model: model.name,
+            start_service_discovery: @start_service_discovery
+          )
         end
 
         def configure_connection