configuration reload fails when using "<<: !include" keyword in yaml configuration

[soukichi]

Describe the bug

configuration reload fails when using "<<: !include" keyword in yaml configuration

To Reproduce

use this configuration:

fluentd.yml

system:
  enable_jit: true
config:
  - source:
      $type: sample
      sample: '{"hello":"world"}'
      auto_increment_key: foo_key
      tag: sample
  - match:
      $type: stdout
      <<: !include output.yml

output.yml

$tag: "**"

exec fluentd

$ fluentd -c fluentd.yml
2022-10-24 14:06:40 +0900 [info]: parsing config file is succeeded path="fluentd.yml"
2022-10-24 14:06:40 +0900 [info]: gem 'fluentd' version '1.15.2'
2022-10-24 14:06:40 +0900 [info]: Oj isn't installed, fallback to Yajl as json parser
2022-10-24 14:06:40 +0900 [warn]: both of Plugin @id and path for <storage> are not specified. Using on-memory store.
2022-10-24 14:06:40 +0900 [warn]: both of Plugin @id and path for <storage> are not specified. Using on-memory store.
2022-10-24 14:06:40 +0900 [warn]: define <match fluent.**> to capture fluentd logs in top level is deprecated. Use <label @FLUENT_LOG> instead
2022-10-24 14:06:40 +0900 [info]: using configuration file: <ROOT>
  <system>
    enable_jit true
  </system>
  <source>
    @type sample
    sample {"hello":"world"}
    auto_increment_key "foo_key"
    tag "sample"
  </source>
  <match **>
    @type stdout
  </match>
</ROOT>
<snip>

exec config reload by USR2 signal
$ kill -USR2 -p 970480

fluentd says

2022-10-24 14:06:46 +0900 [info]: Reloading new config
2022-10-24 14:06:46 +0900 [error]: Failed to reload config file: expected '>' at fluentd.yml line 11,30
 10:       $type: stdout
 11:       <<: !include output.yml

     ------------------------------^

Expected behavior

The include keyword should be expanded as at startup

Your Environment

- Fluentd version:1.15.2
- Operating system:Ubuntu 22.04
- Kernel version:5.15.0-40

Your Configuration

system:
  enable_jit: true
config:
  - source:
      $type: sample
      sample: '{"hello":"world"}'
      auto_increment_key: foo_key
      tag: sample
  - match:
      $type: stdout
      <<: !include output.yml

output.yml> 
$tag: "**"

Your Error Log

2022-10-24 14:06:46 +0900 [error]: Failed to reload config file: expected '>' at ff.yml line 11,30
 10:       $type: stdout
 11:       <<: !include output.yml

     ------------------------------^

Additional context

No response