Support "warning" Severity in addition to "warn"

[daipom]

Issue: #41

We should use "warning" since "warn" is deprecated.

• https://linux.die.net/man/5/syslog.conf

As I commented in the code, I want to fix the upstream library syslog_protocol, but since the PR is already there and the library has not been maintained for a while.
So I temporarily fixed this on the plugin side.

To Reproduce:

  <source>
    @type sample
    tag "test"
    sample {"message":"Hello world."}
    rate 1
  </source>
  <match test.**>
    @type remote_syslog
    host "localhost"
    port 22222
    severity "warning"
    <buffer>
      flush_mode immediate
    </buffer>
  </match>
  <source>
    @type syslog
    tag "receive"
    port 22222
    bind "localhost"
  </source>
  <match receive.**>
    @type stdout
  </match>

Error:

2022-09-28 11:52:32 +0900 [warn]: #0 got unrecoverable error in primary and no secondary error_class=ArgumentError error="'warning' is not a designated severity"
  2022-09-28 11:52:32 +0900 [warn]: #0 /home/daipom/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/syslog_protocol-0.9.2/lib/syslog_protocol/packet.rb:72:in `severity='
  2022-09-28 11:52:32 +0900 [warn]: #0 /home/daipom/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/remote_syslog_sender-1.2.2/lib/remote_syslog_sender/sender.rb:45:in `block (2 levels) in transmit'
  2022-09-28 11:52:32 +0900 [warn]: #0 /home/daipom/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/remote_syslog_sender-1.2.2/lib/remote_syslog_sender/sender.rb:44:in `each'
  2022-09-28 11:52:32 +0900 [warn]: #0 /home/daipom/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/remote_syslog_sender-1.2.2/lib/remote_syslog_sender/sender.rb:44:in `block in transmit'
  2022-09-28 11:52:32 +0900 [warn]: #0 /home/daipom/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/remote_syslog_sender-1.2.2/lib/remote_syslog_sender/sender.rb:37:in `each'
  2022-09-28 11:52:32 +0900 [warn]: #0 /home/daipom/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/remote_syslog_sender-1.2.2/lib/remote_syslog_sender/sender.rb:37:in `transmit'
  2022-09-28 11:52:32 +0900 [warn]: #0 /home/daipom/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/fluent-plugin-remote_syslog-1.0.0/lib/fluent/plugin/out_remote_syslog.rb:105:in `block (2 levels) in write'
  2022-09-28 11:52:32 +0900 [warn]: #0 /home/daipom/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/fluent-plugin-remote_syslog-1.0.0/lib/fluent/plugin/out_remote_syslog.rb:104:in `each_line'
  2022-09-28 11:52:32 +0900 [warn]: #0 /home/daipom/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/fluent-plugin-remote_syslog-1.0.0/lib/fluent/plugin/out_remote_syslog.rb:104:in `block in write'
  2022-09-28 11:52:32 +0900 [warn]: #0 /home/daipom/work/fluentd/fluentd/lib/fluent/plugin/buffer/memory_chunk.rb:81:in `open'
  2022-09-28 11:52:32 +0900 [warn]: #0 /home/daipom/work/fluentd/fluentd/lib/fluent/plugin/buffer/memory_chunk.rb:81:in `open'
  2022-09-28 11:52:32 +0900 [warn]: #0 /home/daipom/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/fluent-plugin-remote_syslog-1.0.0/lib/fluent/plugin/out_remote_syslog.rb:103:in `write'
  2022-09-28 11:52:32 +0900 [warn]: #0 /home/daipom/work/fluentd/fluentd/lib/fluent/plugin/output.rb:1180:in `try_flush'
  2022-09-28 11:52:32 +0900 [warn]: #0 /home/daipom/work/fluentd/fluentd/lib/fluent/plugin/output.rb:1501:in `flush_thread_run'
  2022-09-28 11:52:32 +0900 [warn]: #0 /home/daipom/work/fluentd/fluentd/lib/fluent/plugin/output.rb:501:in `block (2 levels) in start'
  2022-09-28 11:52:32 +0900 [warn]: #0 /home/daipom/work/fluentd/fluentd/lib/fluent/plugin_helper/thread.rb:78:in `block in thread_create'
2022-09-28 11:52:32 +0900 [warn]: #0 bad chunk is moved to /tmp/fluent/backup/worker0/object_bcc/5e9b3da2864b66dc02914ecefd85921a.log
2022-09-28 11:52:33 +0900 [warn]: #0 got unrecoverable error in primary and no secondary error_class=ArgumentError error="'warning' is not a designated severity"

[fujimotos]

LGTM