NGINX作为反向代理时，记录后端接口响应耗时

编辑 /etc/nginx/nginx.conf，在 http 那块配置里面，加上如下代码：

log_format timed_combined '$remote_addr - $remote_user [$time_local]  '
                          '"$request" $status $body_bytes_sent '
                          '"$http_referer" "$http_user_agent"  request_time: $request_time  upstream_response_time: $upstream_response_time';

然后，在具体的 nginx server 配置里的 access_log 后面，加上一句 timed_combined 即可：

access_log /path/to/access.log timed_combined;

|$request_time|
    request processing time in seconds with a milliseconds resolution;
    time elapsed between the first bytes were read from the client and
    the log write after the last bytes were sent to the client

|$request_time指的就是从接受用户请求数据到发送完回复数据的时间。|

http://nginx.org/en/docs/http/ngx_http_upstream_module.html#variables

|$upstream_response_time|
    keeps servers response times in seconds with a milliseconds
    resolution. Several responses are also separated by commas and colons.

|$upstream_response_time说的有点模糊，它指的是从Nginx向后端建立连接开始 
到接受完数据然后关闭连接为 止的时间。||因为会有重试，||它可能有多个时间 
段。一般来说，||$upstream_response_time 会比||$request_time时间短。|

对于HTTP POST的请求，两者相差特别大。因为Nginx会把HTTP request body缓存 
住，接受完毕后才会把数据一起发给后端。

参考资料：

https://nginx.org/en/docs/http/ngx_http_log_module.html#log_format