日志语法
参考: http://nginx.org/en/docs/http/ngx_http_log_module.html
Syntax: access_log path [format [buffer=size] [gzip[=level]] [flush=time] [if=condition]];
access_log off;
Default:
access_log logs/access.log combined;
Context: http, server, location, if in location, limit_except