You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: output.rb
    path of file: ./repos/fluentd/lib/fluent/compat
    
    the code of the file until where you have to start completion: #
# Fluentd
#
#    Licensed under
    
      def format_stream(tag, es)
        # this method will not be used except for the case that plugin calls super
        out = ''
        es.each do |time, record|
          out << format(tag, time, record)
        end
        out
      end