Make login_counter instance variable instead of class one.

app/controllers/health_controller.rb

-require 'prometheus/client/formats/text'
-
 class HealthController < ActionController::Base
   protect_from_forgery with: :exception
   include RequiresHealthToken

app/controllers/metrics_controller.rb

 class MetricsController < ActionController::Base
+  include RequiresHealthToken
+
   protect_from_forgery with: :exception
+
   before_action :validate_prometheus_metrics
-  include RequiresHealthToken
 
   def metrics
     response = "#{metrics_service.health_metrics_text}\n#{metrics_service.prometheus_metrics_text}"

app/controllers/sessions_controller.rb

@@ -47,7 +47,7 @@ class SessionsController < Devise::SessionsController
 
   private
 
-  def self.login_counter
+  def login_counter
     @login_counter ||= Gitlab::Metrics.counter(:user_session_logins, 'User logins count')
   end
 
@@ -129,7 +129,7 @@ class SessionsController < Devise::SessionsController
   end
 
   def log_user_activity(user)
-    SessionsController.login_counter.increment
+    login_counter.increment
     Users::ActivityService.new(user, 'login').execute
   end
 

db/migrate/20170519102115_add_prometheus_settings_to_metrics_settings.rb

 class AddPrometheusSettingsToMetricsSettings < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
+
   disable_ddl_transaction!
 
   DOWNTIME = false