doc: server timing (#1115)

Author: clarkdo

en/api/configuration-server.md

@@ -16,6 +16,7 @@ export default {
   server: {
     port: 8000, // default: 3000
     host: '0.0.0.0', // default: localhost,
+    timing: false
   }
 }
 ```
@@ -47,3 +48,55 @@ export default {
   }
 }
 ```
+
+## timing
+
+- Type: `Object` or `Boolean`
+- Default: `false`
+
+Enabled `server.timing` option (disabled by default) adds a middleware to measure SSR generate time and add it to headers ('Server-Timing')
+
+### Example using timing configuration
+
+`server.timing` can be an object for providing options. Currently supported one is total.
+
+```js
+export default {
+  server: {
+    timing: {
+      total: true
+    }
+  }
+}
+```
+
+### Using timing api
+
+`timing` api is also injected into `response` in server side when `server.time` is enabled.
+
+#### Syntax
+
+```js
+res.timing.start(name, description)
+res.timing.end(name)
+```
+
+#### Example using timing in servermiddleware
+
+```js
+export default function (req, res, next) {
+  res.timing.start('midd', 'Middleware timing description')
+  // server side operation..
+  // ...
+  res.timing.end('midd')
+  next()
+}
+```
+
+Then `server-timing` head will be included in response herder like:
+
+```bash
+Server-Timing: midd;desc="Middleware timing description";dur=2.4
+```
+
+Please refer [Server-Timing MDN](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Server-Timing) for more details.