docs: update for prometheus metric expiration (#11795)

Author: kayx23

conf/config.yaml.example

@@ -611,8 +611,9 @@ plugin_attr:          # Plugin attributes
     #   - 100
     #   - 200
     #   - 500
-    # expire: 0                       # The expiration time after metrics become inactive, unit: second.
-                                      # 0 means the metrics will not expire
+    # expire: 0                       # The expiration time of metrics in seconds.
+                                      # 0 means the metrics will not expire.
+                                      # Only affect apisix_http_status, apisix_bandwidth, and apisix_http_latency.
                                       # If you need to set the expiration time, it is recommended to use 600, which is 10 minutes.
   server-info:                        # Plugin: server-info
     report_ttl: 60                    # Set the TTL in seconds for server info in etcd.

docs/en/latest/plugins/prometheus.md

@@ -96,6 +96,18 @@ plugin_attr:
       - 505
 ```
 
+### Specifying `expire`
+
+`expire` sets the expiration time of `apisix_http_status`, `apisix_bandwidth`, and `apisix_http_latency` metrics in seconds. When set to 0, metrics will not expire.
+
+Here is a configuration example:
+
+```yaml title="conf/config.yaml"
+plugin_attr:
+  prometheus:
+    expire: 86400
+```
+
 ## Metrics endpoint
 
 This Plugin will add the metrics endpoint `/apisix/prometheus/metrics` or your custom export URI for exposing the metrics.

docs/zh/latest/plugins/prometheus.md

@@ -43,7 +43,7 @@ description:  本文将介绍 API 网关 Apache APISIX 如何通过 prometheus 
 
 :::
 
-## 如何修改暴露指标的 uri
+### 如何修改暴露指标的 `export_uri`
 
 你可以在配置文件 `./conf/config.yaml` 的 `plugin_attr` 列表下修改默认的 URI。
 
@@ -78,6 +78,18 @@ plugin_attr:
       - 505
 ```
 
+### 如何修改指标的 `expire`
+
+`expire` 用于设置 `apisix_http_status`、`apisix_bandwidth` 和 `apisix_http_latency` 指标的过期时间（以秒为单位）。当设置为 0 时，指标不会过期。
+
+配置示例如下：
+
+```yaml title="conf/config.yaml"
+plugin_attr:
+  prometheus:
+    expire: 86400
+```
+
 ## API
 
 `prometheus` 插件会增加 `/apisix/prometheus/metrics` 接口或者你自定义的 URI 来暴露其指标信息。