こんな感じかな

my_fluent_logger.rb

# -*- coding: utf-8 -*-
require 'fluent-logger'
require 'active_support/core_ext'
require 'forwardable'

module Mosson
  module FluentLogger
    module ModuleMethods
      def new(file = Rails.root.join("config", "fluent-logger.yml.erb"))
        ActiveSupport::TaggedLogging.new(logger(file))
      end

      def config(file)
        YAML.load(ERB.new(file.read).result)[Rails.env]
      end

      def settings(file)
        config(file).slice('tag', 'host', 'port', 'type')
      end

      def level
        ::Logger::SEV_LABEL.index(Rails.application.config.log_level.to_s.upcase)
      end

      def logger(file)
        Mosson::Logger.new(settings(file), level)
      end
    end
    
    extend ModuleMethods
  end
  
  class Logger < ActiveSupport::Logger
    extend Forwardable

    # logger.rb L.237
    # attr_accessor :level

    attr_reader :port, :host, :type, :tag, :severity, :messages
    def_delegators :fluent_logger,
                   :post,
                   :close

    def initialize(options, level)
      @host, @port, @tag = options[:host], optins[:port], options[:tag]
      @type = (options[:messages_type] || :array).to_sym
      @level = level
      @severity, @messages = 0, []
    end

    def fluent_logger
      @fluent_logger ||= ::Fluent::Logger::FluentLogger.new(
        nil,
        host: host,
        port: port
      )
    end

    # === super: Logger#add
    # === Args
    #
    # +severity+::
    #   Severity.  Constants are defined in Logger namespace: +DEBUG+, +INFO+,
    #   +WARN+, +ERROR+, +FATAL+, or +UNKNOWN+.
    # +message+::
    #   The log message.  A String or Exception.
    # +progname+::
    #   Program name string.  Can be omitted.  Treated as a message if no
    #   +message+ and +block+ are given.
    # +block+::
    #   Can be omitted.  Called to get a message string if +message+ is nil.
    #
    # === Return
    #
    # When the given severity is not high enough (for this particular logger),
    # log no message, and return +true+.

    def add(_severity, message = nil, progname = nil, &block)
      return true if severity < level
      message = (block_given? ? block.call : progname ) if message.blank?
      add_message(message)
      true
    end

    def add_message(message)
      if message.to_s.encoding == Encoding::UTF_8
        messages << message
      else
        messages << message.dup.force_encoding(Encoding::UTF_8)
      end
    end

    def flush
      return if messages.empty?

      post(
        tag,
        message: post_messages,
        level: format_severity(severity)
      )

      @severity = 0
      messages.clear
    end

    def post_messages
      if @messages_type == :string
       messages.join("\n")
     else
       messages
     end
    end

    def format_severity(severity)
      ::Logger::SEV_LABEL[severity] || 'ANY'
    end
  end
end