Collect features_json.log

@shinya.maeda comments in gitlab-com/runbooks!3012 (comment 459430623)

Maybe you can help here? I want to see the features_json.log in Kibana, but have no idea where to change in this project. Thanks!

This will need to be added to the rails config https://ops.gitlab.net/gitlab-cookbooks/gitlab_fluentd/-/blob/master/templates/default/rails.conf.erb if we want to to be added to the rails index.

@shinya.maeda If we send it to an existing ES index, we will need to make sure there are no conflicting fields. For example, a problem might be if we have a field in this log that is a string vs a field in the production_json log with a type of integer with the same name.

I also don't see this log yet in production, do we need to turn it on first?

cc @AnthonySandoval for scheduling.