Request context functionality - it's a thread-safe dictionary available globally, and initialised during `process_action`. It is used primarily to provide context to ActionMailer log subscribers.

The default field added is request_id which is automatically generated by rails.

Additional fields can be added by using:

LogStasher.add_custom_fields_to_request_context do |fields|
  fields[:key] = value
end

The block above is executed in a callback, after a request has been completed.