Execute metrics initializer earlier

This makes sure that Gitlab::Metrics::RackMiddleware is added before
Gitlab::EtagCaching::Middleware.

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/metrics.rb`.
+`config/initializers/8_metrics.rb`.
 ### Examples

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

 require 'spec_helper'
-require_relative '../../config/initializers/metrics'
+require_relative '../../config/initializers/8_metrics'
 describe 'instrument_classes', lib: true do
   let(:config) { double(:config) }