Avoid using Send (but is it better?) + small rubocop cleanup

lib/gitlab/metrics/prometheus.rb

@@ -53,7 +53,7 @@ module Gitlab
         return metric if metric
 
         PROVIDER_MUTEX.synchronize do
-          provide_metric(name) || registry.send(method, name, *args)
+          provide_metric(name) || registry.method(method).call(name, *args)
         end
       end
 

lib/gitlab/middleware/rails_queue_duration.rb

@@ -5,6 +5,13 @@
 module Gitlab
   module Middleware
     class RailsQueueDuration
+      def self.metric_rails_queue_duration_seconds
+        @metric_rails_queue_duration_seconds ||= Gitlab::Metrics.histogram(
+          :gitlab_rails_queue_duration_seconds,
+          Gitlab::Metrics::Transaction::BASE_LABELS
+        )
+      end
+
       def initialize(app)
         @app = app
       end
@@ -21,15 +28,6 @@ module Gitlab
 
         @app.call(env)
       end
-
-      private
-
-      def self.metric_rails_queue_duration_seconds
-        @metric_rails_queue_duration_seconds ||= Gitlab::Metrics.histogram(
-          :gitlab_rails_queue_duration_seconds,
-          Gitlab::Metrics::Transaction::BASE_LABELS
-        )
-      end
     end
   end
 end