$request_time
官方解釋：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.
客戶端發出第一個字節開始到接收到最后一個字節的時間。
$upstream_response_time
官方解釋：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與后端建立連接到傳輸完數據連接斷開為止的時間。
$upstream_connect_time
官方解釋：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與上游服務器建立連接所消耗的時間，其中包含連接握手的時間，單位為秒。
$upstream_header_time
官方解釋：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.
從上游服務器接收響應頭的時間，單位為秒。