emit backtrace on UNRECOVERABLE_ERRORS

[Etsukata]

UNRECOVERABLE_ERRORS are thought to be rather critical, so we will
benefit from backtrace output.

Originally emitted in fluentd v1.3 or earlier, but suppressed in
c011256bec95eca

Signed-off-by: Eiichi Tsukata devel@etsukata.com

[Etsukata]

Example output log

• before

2018-10-09 11:52:13 +0900 [warn]: #0 got unrecoverable error in primary and no secondary error_class=NoMethodError error="undefined method `has_key?' for #<String:0x00007f0ee9f4bf60>" 
2018-10-09 11:52:13 +0900 [warn]: #0 bad chunk is moved to /tmp/fluent/backup/worker0/object_3f8775b4a618/577c2d31e98de1fc80472d4d2edd9c7c.log

• after

2018-10-11 09:00:04 +0900 [warn]: #0 got unrecoverable error in primary and no secondary error_class=NoMethodError error="undefined method `has_key?' for #<String:0x00007fdd141c22a0>" 
  2018-10-11 09:00:04 +0900 [warn]: #0 /opt/td-agent/embedded/lib/ruby/gems/2.4.0/gems/fluent-plugin-elasticsearch-2.10.3/lib/fluent/plugin/elasticsearch_error_handler.rb:66:in `block in handle_error'
  2018-10-11 09:00:04 +0900 [warn]: #0 /opt/td-agent/embedded/lib/ruby/gems/2.4.0/gems/fluentd-1.2.2/lib/fluent/event.rb:323:in `each'
  2018-10-11 09:00:04 +0900 [warn]: #0 /opt/td-agent/embedded/lib/ruby/gems/2.4.0/gems/fluentd-1.2.2/lib/fluent/event.rb:323:in `block in each'
  2018-10-11 09:00:04 +0900 [warn]: #0 /opt/td-agent/embedded/lib/ruby/gems/2.4.0/gems/fluentd-1.2.2/lib/fluent/plugin/buffer/memory_chunk.rb:80:in `open'
  2018-10-11 09:00:04 +0900 [warn]: #0 /opt/td-agent/embedded/lib/ruby/gems/2.4.0/gems/fluentd-1.2.2/lib/fluent/plugin/buffer/memory_chunk.rb:80:in `open'
  2018-10-11 09:00:04 +0900 [warn]: #0 /opt/td-agent/embedded/lib/ruby/gems/2.4.0/gems/fluentd-1.2.2/lib/fluent/event.rb:322:in `each'
  2018-10-11 09:00:04 +0900 [warn]: #0 /opt/td-agent/embedded/lib/ruby/gems/2.4.0/gems/fluent-plugin-elasticsearch-2.10.3/lib/fluent/plugin/elasticsearch_error_handler.rb:27:in `handle_error'
  2018-10-11 09:00:04 +0900 [warn]: #0 /opt/td-agent/embedded/lib/ruby/gems/2.4.0/gems/fluent-plugin-elasticsearch-2.10.3/lib/fluent/plugin/out_elasticsearch.rb:516:in `send_bulk'
  2018-10-11 09:00:04 +0900 [warn]: #0 /opt/td-agent/embedded/lib/ruby/gems/2.4.0/gems/fluent-plugin-elasticsearch-2.10.3/lib/fluent/plugin/out_elasticsearch.rb:414:in `write'
  2018-10-11 09:00:04 +0900 [warn]: #0 /opt/td-agent/embedded/lib/ruby/gems/2.4.0/gems/fluentd-1.2.2/lib/fluent/plugin/output.rb:1099:in `try_flush'
  2018-10-11 09:00:04 +0900 [warn]: #0 /opt/td-agent/embedded/lib/ruby/gems/2.4.0/gems/fluentd-1.2.2/lib/fluent/plugin/output.rb:1379:in `flush_thread_run'
  2018-10-11 09:00:04 +0900 [warn]: #0 /opt/td-agent/embedded/lib/ruby/gems/2.4.0/gems/fluentd-1.2.2/lib/fluent/plugin/output.rb:440:in `block (2 levels) in start'
  2018-10-11 09:00:04 +0900 [warn]: #0 /opt/td-agent/embedded/lib/ruby/gems/2.4.0/gems/fluentd-1.2.2/lib/fluent/plugin_helper/thread.rb:78:in `block in thread_create'
2018-10-11 09:00:04 +0900 [warn]: #0 bad chunk is moved to /tmp/fluent/backup/worker0/object_3fee8de6d690/577e8a7043da108ac692cde87efa8b6d.log

[repeatedly]

Merged. Thanks!