logrageを使う

Railsのログをjson化するためにlogrageを使う

Gemfile

gem 'lograge'
gem 'logstash-event' # json化するために必要

config/initializers/lograge.rb

Rails.application.configure do
  config.lograge.enabled = true

  # If you're using Rails 5's API-only mode and inherit from ActionController::API, you must define it as the controller base class which lograge will patch:
  # config.lograge.base_controller_class = 'ActionController::API'

  # JSON(logstash形式)で出力
  config.lograge.formatter = Lograge::Formatters::Logstash.new

  # オリジナルのlogを残す
  config.lograge.keep_original_rails_log = true

  # logrageのlogを別ファイルに保存
  config.lograge.logger = ActiveSupport::Logger.new "#{Rails.root}/log/lograge_#{Rails.env}.log"

  # 独自パラメータを取得 (controllerが落ちてくる)
  config.lograge.custom_payload do |controller|
    params = controller.request.params.except(* %w[controller action])

    {
      request_id: controller.request.request_id,
      user_id: controller.try(:current_user).try(:id), # 例) 確定してるなら&.のほうが良い
      params: params
    }
  end

  # 独自パラメータを設定 (ActiveSupport::Notifications::Event name='process_action.action_controller' が落ちてくる)
  config.lograge.custom_options = lambda do |event|
    ret = {
      request_ip: event.payload[:request_ip],
      error_message: nil,
      error_stacktrace: nil
    }

    if (eo = event.payload[:exception_object])
      ret.merge!(
        error_message: eo.message,
        error_stacktrace: eo.backtrace.join("\n")
      )
    end

    ret
  end
end

こんなログが出力される

#{Rails.root}/log/lograge_#{Rails.env}.log

正常時
{"method":"GET","path":"/api/workflows","format":"html","controller":"Api::WorkflowsController","action":"index","status":200,"duration":58.42,"view":11.2,"db":40.94,"request_ip":null,"exception":null,"exception_object":null,"error_message":null,"error_stacktrace":null,"request_id":"4dbf2f3c-ff83-422e-b16d-d8584cbdcf6e","user_id":null,"params":{},"@timestamp":"2019-04-07T04:51:39.978Z","@version":"1","message":"[200] GET /api/workflows (Api::WorkflowsController#index)"}

例外時
{"method":"GET","path":"/api/workflows","format":"html","controller":"Api::WorkflowsController","action":"index","status":500,"error":"RuntimeError: ","duration":0.63,"view":0.0,"db":0.0,"request_ip":null,"exception":null,"exception_object":null,"error_message":"","error_stacktrace":"<<<ここにスタックトレースが出力>>>","request_id":"b680ad4a-9eea-4392-9084-2df09d0ab850","user_id":null,"params":{},"@timestamp":"2019-04-07T04:55:13.013Z","@version":"1","message":"[500] GET /api/workflows (Api::WorkflowsController#index)"}

参考:
https://github.com/roidrage/lograge

custom_optionsで落ちてくるeventは 'process_action.action_controller'
https://edgeguides.rubyonrails.org/active_support_instrumentation.html#process-action-action-controller