logger: force_stacktrace_level was not applied to PluginLogger (#5038)

**Which issue(s) this PR fixes**: 
* Fixes a bug in #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:

```xml
<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`.

Signed-off-by: Daijiro Fukuda <[email protected]>

Author: daipom

lib/fluent/log.rb

@@ -174,6 +174,7 @@ def dup
       clone.format = @format
       clone.time_format = @time_format
       clone.log_event_enabled = @log_event_enabled
+      clone.force_stacktrace_level(@forced_stacktrace_level)
       # optional headers/attrs are not copied, because new PluginLogger should have another one of it
       clone
     end
@@ -652,6 +653,9 @@ def initialize(logger)
       if logger.instance_variable_defined?(:@suppress_repeated_stacktrace)
         @suppress_repeated_stacktrace = logger.instance_variable_get(:@suppress_repeated_stacktrace)
       end
+      if logger.instance_variable_defined?(:@forced_stacktrace_level)
+        @forced_stacktrace_level = logger.instance_variable_get(:@forced_stacktrace_level)
+      end
       if logger.instance_variable_defined?(:@ignore_repeated_log_interval)
         @ignore_repeated_log_interval = logger.instance_variable_get(:@ignore_repeated_log_interval)
       end

test/test_log.rb

@@ -811,14 +811,6 @@ def test_initialize
     assert_equal(logger, @logger)
   end
 
-  def test_level
-    log = Fluent::PluginLogger.new(@logger)
-    assert_equal(log.level, @logger.level)
-    log.level = "fatal"
-    assert_equal(Fluent::Log::LEVEL_FATAL, log.level)
-    assert_equal(Fluent::Log::LEVEL_TRACE, @logger.level)
-  end
-
   def test_enable_color
     log = Fluent::PluginLogger.new(@logger)
     log.enable_color(true)
@@ -864,6 +856,45 @@ def test_log_types
     @logger.fatal :worker0, "fatal log 6"
   end
 
+  sub_test_case "take over the parent logger" do
+    def test_level
+      log = Fluent::PluginLogger.new(@logger)
+      assert_equal(log.level, @logger.level)
+      log.level = "fatal"
+      assert_equal(Fluent::Log::LEVEL_FATAL, log.level)
+      assert_equal(Fluent::Log::LEVEL_TRACE, @logger.level)
+    end
+
+    def test_options
+      parent_log = Fluent::Log.new(
+        ServerEngine::DaemonLogger.new(
+          @log_device,
+          log_level: ServerEngine::DaemonLogger::INFO,
+        ),
+        suppress_repeated_stacktrace: true,
+        ignore_repeated_log_interval: 10,
+        ignore_same_log_interval: 10,
+      )
+      parent_log.force_stacktrace_level(Fluent::Log::LEVEL_INFO)
+
+      log = Fluent::PluginLogger.new(parent_log)
+      assert_equal(
+        [
+          true,
+          Fluent::Log::LEVEL_INFO,
+          10,
+          10,
+        ],
+        [
+          log.instance_variable_get(:@suppress_repeated_stacktrace),
+          log.instance_variable_get(:@forced_stacktrace_level),
+          log.instance_variable_get(:@ignore_repeated_log_interval),
+          log.instance_variable_get(:@ignore_same_log_interval),
+        ]
+      )
+    end
+  end
+
   sub_test_case "supervisor process type" do
     setup do
       dl_opts = {}
@@ -1114,6 +1145,49 @@ def test_terminate
     mock(plugin.log).reset
     plugin.terminate
   end
+
+  sub_test_case "take over the parent logger" do
+    def setup
+      super
+
+      begin
+        saved_global_logger = $log
+        $log = Fluent::Log.new(
+          ServerEngine::DaemonLogger.new(
+            Fluent::Test::DummyLogDevice.new,
+            log_level: ServerEngine::DaemonLogger::INFO,
+          ),
+          suppress_repeated_stacktrace: true,
+          ignore_repeated_log_interval: 10,
+          ignore_same_log_interval: 10,
+        )
+        $log.force_stacktrace_level(Fluent::Log::LEVEL_INFO)
+        yield
+      ensure
+        $log = saved_global_logger
+      end
+    end
+
+    def test_options
+      plugin = DummyPlugin.new
+      plugin.configure(Fluent::Config::Element.new("input", "", {"@id" => "foo"}, []))
+
+      assert_equal(
+        [
+          true,
+          Fluent::Log::LEVEL_INFO,
+          10,
+          10,
+        ],
+        [
+          plugin.log.instance_variable_get(:@suppress_repeated_stacktrace),
+          plugin.log.instance_variable_get(:@forced_stacktrace_level),
+          plugin.log.instance_variable_get(:@ignore_repeated_log_interval),
+          plugin.log.instance_variable_get(:@ignore_same_log_interval),
+        ]
+      )
+    end
+  end
 end
 
 class LogDeviceIOTest < Test::Unit::TestCase