Output exception stacktrace as part of @FLUENT_LOG

**Problem**

Fluentd has "stacktrace" support, which outputs the detailed execution context
whenever an unhandled exception happened. This feature is very useful when
investigating errors occured on production system.

Here is an example:

```
2020-01-01 00:00:00 +0000 [warn]: #0 emit transaction failed: error_class=IOError error="closed stream" location="/fluentd/vendor/bundle/ruby/2.6.0/gems/fluentd-1.11.1/lib/fluent/plugin/buffer/file_chunk.rb:82:in `pos'" tag="cafiscode-eks-cluster.default"
2020-01-01 00:00:00 +0000 [warn]: #0 /fluentd/vendor/bundle/ruby/2.7.0/gems/fluentd-1.11.1/lib/fluent/plugin/buffer/file_chunk.rb:82:in `pos'
2020-01-01 00:00:00 +0000 [warn]: #0 /fluentd/vendor/bundle/ruby/2.7.0/gems/fluentd-1.11.1/lib/fluent/plugin/buffer/file_chunk.rb:82:in `rollback'
2020-01-01 00:00:00 +0000 [warn]: #0 /fluentd/vendor/bundle/ruby/2.7.0/gems/fluentd-1.11.1/lib/fluent/plugin/buffer.rb:339:in `rescue in block in write'

```

The problem is that these stacktraces won't never be emitted to `@FLUENT_LOG`.

**Desired Behavior**

* Fluentd should emit the stacktrace to `@FLUENT_LOG` as well.