logger: force_stacktrace_level was not applied to PluginLogger

[daipom]

Which issue(s) this PR fixes:

• Fixes a bug in log: add system_config option to force stacktrace level #5008

What this PR does / why we need it:
In #5008, we have added a new feature force_stacktrace_level.
However, it was not applied to PluginLogger.
So, depending on the settings and error locations, force_stacktrace_level could not work.
This PR fixes it.

Docs Changes:
Not needed because #5008 is not released yet.

Release Note:
Not needed because #5008 is not released yet.

How to reproduce the bug:

conf:

<system>
  <log>
    forced_stacktrace_level info
  </log>
</system>

<source>
  @type sample
  @id test
  tag test.fail
</source>

<match test.fail>
  @type null
  @id null
  never_flush
  <buffer>
    flush_mode interval
    flush_interval 1s
  </buffer>
</match>

• Set @id to use PluginLogger.
• Use buffer to use PluginLogger.
  • Without buffer, RootAgent outputs the logs for flushing errors by using the global logger. (RootAgent#handle_emits_error)

log:

2025-07-29 12:39:36 +0900 [info]: #0 fluentd worker is now running worker=0
2025-07-29 12:39:38 +0900 [warn]: #0 [null] failed to flush the buffer. retry_times=0 next_retry_time=2025-07-29 12:39:39 +0900 chunk="63b09284c334efa1927547b5623268ff" error_class=RuntimeError error="failed to flush"
  2025-07-29 12:39:38 +0900 [warn]: #0 /home/daipom/work/fluentd/fluentd/lib/fluent/plugin/out_null.rb:59:in `write'
  2025-07-29 12:39:38 +0900 [warn]: #0 /home/daipom/work/fluentd/fluentd/lib/fluent/plugin/output.rb:1212:in `try_flush'
  2025-07-29 12:39:38 +0900 [warn]: #0 /home/daipom/work/fluentd/fluentd/lib/fluent/plugin/output.rb:1527:in `flush_thread_run'
  2025-07-29 12:39:38 +0900 [warn]: #0 /home/daipom/work/fluentd/fluentd/lib/fluent/plugin/output.rb:490:in `block (2 levels) in start'
  2025-07-29 12:39:38 +0900 [warn]: #0 /home/daipom/work/fluentd/fluentd/lib/fluent/plugin_helper/thread.rb:78:in `block in thread_create'

• The log levels of stacktraces are warn, although forced_stacktrace_level is info.

[Watson1978]

👍🏻

[kenhys]

LGTM.