Configure OmniAuth to use GitLab AppLogger

OmniAuth logger was not being configured properly and some logs
were being dropped. This change ensures OmniAuth log messages
are output to `application.log` and/or `application_json.log`
as appropriate depending on configuration.

Changelog: security

config/initializers/omniauth.rb

@@ -19,6 +19,4 @@ OmniAuth.config.before_request_phase do |env|
   Gitlab::RequestForgeryProtection.call(env)
 end
 
-# Use json formatter
-OmniAuth.config.logger.formatter = Gitlab::OmniauthLogging::JSONFormatter.new
-OmniAuth.config.logger.level = Logger::ERROR if Rails.env.production?
+OmniAuth.config.logger = Gitlab::AppLogger

lib/gitlab/omniauth_logging/json_formatter.rb

-# frozen_string_literal: true
-
-require 'json'
-
-module Gitlab
-  module OmniauthLogging
-    class JSONFormatter
-      def call(severity, datetime, progname, msg)
-        { severity: severity, timestamp: datetime.utc.iso8601(3), pid: $$, progname: progname, message: msg }.to_json << "\n"
-      end
-    end
-  end
-end

spec/lib/gitlab/omniauth_logging/json_formatter_spec.rb

-# frozen_string_literal: true
-
-require 'spec_helper'
-
-RSpec.describe Gitlab::OmniauthLogging::JSONFormatter do
-  it "generates log in json format" do
-    Timecop.freeze(Time.utc(2019, 12, 04, 9, 10, 11, 123456)) do
-      expect(subject.call(:info, Time.now, 'omniauth', 'log message'))
-        .to eq %Q({"severity":"info","timestamp":"2019-12-04T09:10:11.123Z","pid":#{Process.pid},"progname":"omniauth","message":"log message"}\n)
-    end
-  end
-end