fix fluentd tags from firelens

fluentd.conf

<match *-firelens-*>
  @type firelens_tag_filter
</match>

<match app.firelens.**>
  # ...
</match>

out_firelens_tag_filter.rb

require "fluent/plugin/output"

class Fluent::Plugin::FirelensTagFilterOutput < Fluent::Plugin::Output
  Fluent::Plugin.register_output('firelens_tag_filter', self)
  helpers :event_emitter

  def process(tag, es)
    rewrited_tag = tag.gsub(/^(.*?)-firelens-(.*)/, '\1.firelens.\2')
    router.emit_stream(rewrited_tag, es)
  end
end