Add backtrace to production_json.log

Sample JSON:

{
  "method": "GET",
  "path": "/admin",
  "format": "html",
  "controller": "Admin::DashboardController",
  "action": "index",
  "status": 500,
  "duration": 2584.11,
  "view": 0,
  "db": 9.21,
  "time": "2019-11-14T13:12:46.156Z",
  "params": [],
  "remote_ip": "127.0.0.1",
  "user_id": 1,
  "username": "root",
  "ua": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:70.0) Gecko/20100101 Firefox/70.0",
  "queue_duration": 274.35,
  "correlation_id": "KjDVUhNvvV3",
  "cpu_s": 2.837645135999999,
  "exception": {
    "class": "NameError",
    "message": "undefined local variable or method `adsf' for #<Admin::DashboardController:0x00007ff3c9648588>",
    "backtrace": [
      "app/controllers/admin/dashboard_controller.rb:11:in `index'",
      "ee/app/controllers/ee/admin/dashboard_controller.rb:14:in `index'",
      "ee/lib/gitlab/ip_address_state.rb:10:in `with'",
      "ee/app/controllers/ee/application_controller.rb:43:in `set_current_ip_address'",
      "lib/gitlab/session.rb:11:in `with_session'",
      "app/controllers/application_controller.rb:450:in `set_session_storage'",
      "app/controllers/application_controller.rb:444:in `set_locale'",
      "ee/lib/gitlab/jira/middleware.rb:19:in `call'"
    ]
  }
}

This change also drops the error field in favor of exception.class and exception.message.

Closes #36297 (closed)