gistfile1.txt

filter {
  ruby {
    path => "${PWD}/processor-hostname.logstash-filter-ruby.rb"
    script_params => {
      "target" => "@processed-by"
    }
  }
}

processor-hostname.logstash-filter-ruby.rb

def register(params)
  params = params.dup
  @target = params.delete('target') || fail(LogStash::ConfigurationError.new('`target` parameter is required'))

  @hostname = Socket.get_hostname.dup.freeze
end

def filter(event)
  event.set(@target, @hostname)
ensure
  return [event]
end