Fix missing active constant error

When we refer to a constant from initializers which is supposed to be
loaded by `ActiveSupport::Dependencies`, it's possible to get an error
in the development environment if Rails tries to load/reload a missing
constant under the namespace of that constant.

The reason is since the constant is loaded in initializers, it stays in
the memory and conflicts with the one reloaded by Rails.

This commit fixes only the `Gitlab::Metrics::Subscribers::ActiveRecord`
case which has been used in lograge configuration.

config/initializers/lograge.rb

 # Only use Lograge for Rails
 unless Gitlab::Runtime.sidekiq?
+  Rails.application.reloader.to_prepare do
     filename = File.join(Rails.root, 'log', "#{Rails.env}_json.log")
+    db_counter = Gitlab::Metrics::Subscribers::ActiveRecord
 
     Rails.application.configure do
       config.lograge.enabled = true
@@ -15,7 +17,7 @@ unless Gitlab::Runtime.sidekiq?
         data[:db_duration_s] = Gitlab::Utils.ms_to_round_sec(data.delete(:db)) if data[:db]
         data[:view_duration_s] = Gitlab::Utils.ms_to_round_sec(data.delete(:view)) if data[:view]
         data[:duration_s] = Gitlab::Utils.ms_to_round_sec(data.delete(:duration)) if data[:duration]
-      data.merge!(::Gitlab::Metrics::Subscribers::ActiveRecord.db_counter_payload)
+        data.merge!(db_counter.db_counter_payload)
 
         # Remove empty hashes to prevent type mismatches
         # These are set to empty hashes in Lograge's ActionCable subscriber
@@ -34,4 +36,5 @@ unless Gitlab::Runtime.sidekiq?
       # Add request parameters to log output
       config.lograge.custom_options = Gitlab::Lograge::CustomOptions
     end
+  end
 end