fluentD to elasticsearch how to take field as string?

[yyhk123]

Hello,
I am sending logs in json format to elasticsearch using fluentD.
There is a field called statusCode, and it both comes in as string and int, but because the data type is inconsistent, it is giving me an error.

how can i make it to take it as string and send as string even when it comes in as int?
i tried lua, but it looks like fluentd does not support.

here is the config file i used:

<system>
   log_level debug
</system>

<source>
  @type tail
  @id in_tail_container_logs
  path /root/fluent/log.log
  pos_file /root/fluent/esb_log2.log.pos
  tag esb_log
  read_from_head true
  <parse>
    @type regexp
    expression /^(?<json>\{.*\})$/
    time_format %Y-%m-%d %H:%M:%S
  </parse>
</source>

<filter esb_log>
  @type parser
  key_name json
  reserve_data true
  <parse>
    @type json
  </parse>
</filter>

<filter esb_log>
  @type record_transformer
  remove_keys json
</filter>

<match esb_log>
  @type elasticsearch
  host 10.111.222.333
  port 9200
  user fluent
  password fluent
  index_name ${tag}
  op_type create
  scheme https
  ssl_verify false
  <buffer>
    flush_mode interval
    flush_interval 5s
  </buffer>
</match>

This works fine if the statusCode is consistent.
few things i tried is inserting this line:

<filter esb_log>
  @type parser
  key_name json
  reserve_data true
  types statusCode:string
  <parse>
    @type json
  </parse>
</filter>

and

<filter esb_log>
  @type record_transformer
  remove_keys json
  <record>
    statusCode "${record['statusCode'].to_s}"
  </record>
</filter>

but they were not so helpful.
please help!!
thank you!