$http_x_forwarded_for field missing from nginx parser plugin format

[yaron-idan]

• fluentd or td-agent version.
  fluentd v1.1.2
• Environment information, e.g. OS.
  Linux / Docker
• Your configuration

<source>
  @type tail
  path "/logs/*.log"
  format nginx
  tag "nginx.*"
</source>

• Your problem explanation. If you have an error logs, write it together.
  When receiving a log from nginx, I get the following message -
  fluentd_1 | 2018-03-22 18:07:49 +0000 [warn]: #0 pattern not match: "172.18.0.1 - - [22/Mar/2018:18:07:49 +0000] \"GET / HTTP/1.1\" 200 612 \"-\" \"curl/7.54.0\" \"-\""
  If I swap the format configuration to the one listed in the plugin docs and add the last field like so the log level goes through -

format /^(?<remote>[^ ]*) (?<host>[^ ]*) (?<user>[^ ]*) \[(?<time>[^\]]*)\] "(?<method>\S+)(?: +(?<path>[^\"]*) +\S*)?" (?<code>[^ ]*) (?<size>[^ ]*)(?: "(?<referer>[^\"]*)" "(?<agent>[^\"]*)" "(?<agent2>[^\"]*)")?$/
  time_format %d/%b/%Y:%H:%M:%S %z

I find this a bit strange since the last field ($http_x_forwarded_for) has been part of the default nginx logging for a long time. Is there a real reason for omitting this line from the plugins format?

If the field needs adding, I'm cool with opening the PR for it. Just wanted to check up before starting to work on it.

[repeatedly]

Maybe, nginx's regexp comes from https://github.com/nginx/nginx/blob/764737256521de5c34f7ce1cc1b27bf440b6cced/src/http/modules/ngx_http_log_module.c#L227
I checked the configuration and $http_x_forwarded_for is included in the example.
https://github.com/nginx/nginx/blob/9207cc84b21e94283478cee7a953b1859c4434cb/conf/nginx.conf#L21
So support $http_x_forwarded_for seems good for many users.

[repeatedly]

v1.2.0.pre1 includes this change.