Always load the metrics the last

Because this could potentially load a model and we shouldn't
load models before all the patches we have in places.

config/initializers/8_metrics.rb → config/initializers/zz_metrics.rb

+# This file was prefixed with zz_ because we want to load it the last!
+# See: https://gitlab.com/gitlab-org/gitlab-ce/issues/55611
+
 # Autoload all classes that we want to instrument, and instrument the methods we
 # need. This takes the Gitlab::Metrics::Instrumentation module as an argument so
 # that we can stub it for testing, as it is only called when metrics are

doc/development/instrumentation.md

@@ -35,7 +35,7 @@ Using this method is in general preferred over directly calling the various
 instrumentation methods.
 
 Method instrumentation should be added in the initializer
-`config/initializers/8_metrics.rb`.
+`config/initializers/zz_metrics.rb`.
 
 ### Examples
 

spec/initializers/8_metrics_spec.rb → spec/initializers/zz_metrics_spec.rb

@@ -16,7 +16,7 @@ describe 'instrument_classes' do
   end
 
   it 'can autoload and instrument all files' do
-    require_relative '../../config/initializers/8_metrics'
+    require_relative '../../config/initializers/zz_metrics'
     expect { instrument_classes(config) }.not_to raise_error
   end
 end