$request_time
官方解释：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.
客户端发出第一个字节开始到接收到最后一个字节的时间。
$upstream_response_time
官方解释：keeps time spent on receiving the response from the upstream server; the time is kept in seconds with millisecond resolution. Times of several responses are separated by commas and colons like addresses in the $upstream_addr variable.
Nginx与后端建立连接到传输完数据连接断开为止的时间。
$upstream_connect_time
官方解释：keeps time spent on establishing a connection with the upstream server (1.9.1); the time is kept in seconds with millisecond resolution. In case of SSL, includes time spent on handshake. Times of several connections are separated by commas and colons like addresses in the $upstream_addr variable.
Nginx与上游服务器建立连接所消耗的时间，其中包含连接握手的时间，单位为秒。
$upstream_header_time
官方解释：keeps time spent on receiving the response header from the upstream server (1.7.10); the time is kept in seconds with millisecond resolution. Times of several responses are separated by commas and colons like addresses in the $upstream_addr variable.
从上游服务器接收响应头的时间，单位为秒。