Add User-Agent to production_json.log

This will help production gain more visibility which browsers may be
having issues.

app/controllers/application_controller.rb

@@ -110,6 +110,7 @@ class ApplicationController < ActionController::Base
   def append_info_to_payload(payload)
     super
 
+    payload[:ua] = request.env["HTTP_USER_AGENT"]
     payload[:remote_ip] = request.remote_ip
 
     logged_user = auth_user

changelogs/unreleased/sh-add-ua-to-lograge-logs.yml

+---
+title: Add User-Agent to production_json.log
+merge_request: 21546
+author:
+type: other

config/initializers/lograge.rb

@@ -22,7 +22,8 @@ unless Sidekiq.server?
         params: params,
         remote_ip: event.payload[:remote_ip],
         user_id: event.payload[:user_id],
-        username: event.payload[:username]
+        username: event.payload[:username],
+        ua: event.payload[:ua]
       }
 
       gitaly_calls = Gitlab::GitalyClient.get_request_count