Change OmniAuth log format to JSON

 - Change OmniAuth log format to JSON
 - Keep only ERROR log in production

changelogs/unreleased/205184-change-omniauth-log-format-to-json.yml

+---
+title: Change OmniAuth log format to JSON
+merge_request: 25086
+author:
+type: other

config/initializers/omniauth.rb

@@ -16,3 +16,7 @@ OmniAuth.config.allowed_request_methods << :get if Gitlab.config.omniauth.auto_s
 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?

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'
+
+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