docs: add missing route label to prometheus latency metric

docs/en/latest/plugins/prometheus.md

@@ -257,6 +257,7 @@ The following metrics are exported by the `prometheus` Plugin:
   | Name     | Description                                                                                                                         |
   |----------|-------------------------------------------------------------------------------------------------------------------------------------|
   | type     | Value can be one of `apisix`, `upstream`, or `request`. This translates to latency caused by APISIX, Upstream, or both (their sum). |
+  | route    | `route_id` of the matched Route with request. Defaults to an empty string if the Routes don't match.                          |
   | service  | `service_id` of the Route matching the request. If the Route does not have a `service_id` configured, it defaults to `$host`.       |
   | consumer | `consumer_name` of the Consumer matching the request. Defaults to an empty string if it does not match.                             |
   | node     | IP address of the Upstream node.                                                                                                    |

docs/zh/latest/plugins/prometheus.md

@@ -231,6 +231,7 @@ scrape_configs:
     | 名称          |    描述                                                                                 |
     | -------------| --------------------------------------------------------------------------------------- |
     | type         | 该值可以是 `apisix`、`upstream` 和 `request`，分别表示耗时的来源是 APISIX、上游以及两者总和。 |
+    | route        | 与请求匹配的路由的 `route_id`，如果未匹配，则默认为空字符串。 |
     | service      | 与请求匹配的路由 的 `service_id`。当路由缺少 `service_id` 时，则默认为 `$host`。             |
     | consumer     | 与请求匹配的消费者的 `consumer_name`。未匹配，则默认为空字符串。                             |
     | node         | 上游节点的 IP 地址。                                                                      |