docs: add bandwidthLimit plugin (#356)

Signed-off-by: spacewander <[email protected]>

site/content/en/docs/reference/plugins/bandwidth_limit.md

@@ -0,0 +1,88 @@
+---
+title: Bandwidth Limit
+---
+
+## Description
+
+The `bandwidthLimit` plugin restricts the maximum bandwidth of the data stream by leveraging Envoy's `bandwidth_limit` filter. Note that this limitation only applies to the request or response body.
+
+## Attribute
+
+|       |         |
+|-------|---------|
+| Type  | Traffic |
+| Order | Outer   |
+
+## Configuration
+
+See the corresponding [Envoy documentation](https://www.envoyproxy.io/docs/envoy/v1.28.0/configuration/http/http_filters/bandwidth_limit_filter).
+
+## Usage
+
+Assumed we have the HTTPRoute below attached to `localhost:10000`, and a backend server listening to port `8080`:
+
+```yaml
+apiVersion: gateway.networking.k8s.io/v1
+kind: HTTPRoute
+metadata:
+  name: default
+spec:
+  parentRefs:
+  - name: default
+    namespace: default
+  rules:
+  - matches:
+    - path:
+        type: PathPrefix
+        value: /
+    backendRefs:
+    - name: backend
+      port: 8080
+```
+
+By applying the configuration below, the upload speed of requests sent to `http://localhost:10000/` will be limited to approximately 10kb/s:
+
+```yaml
+apiVersion: mosn.io/v1
+kind: HTTPFilterPolicy
+metadata:
+  name: policy
+spec:
+  targetRef:
+    group: gateway.networking.k8s.io
+    kind: HTTPRoute
+    name: default
+  filters:
+    bandwidthLimit:
+      config:
+        statPrefix: policy_bandwidth_limit
+        enableMode: REQUEST
+        limitKbps: 10
+        fillInterval: 0.02s
+```
+
+The bandwidth limit is not very precise. The smaller the `fillInterval`, the higher the accuracy.
+
+To test the effect of the plugin, we have the backend server listening on port `8080` which echoes all received requests. Let's try it with [bombardier](https://pkg.go.dev/github.com/codesenberg/bombardier):
+
+```
+$ bombardier -m POST -f go.sum -c 10 -t 180s -d 60s -l http://localhost:10000/
+Bombarding http://localhost:10000/ for 1m0s using 10 connection(s)
+[======================================================================================================================================================] 1m0s
+Done!
+Statistics        Avg      Stdev        Max
+  Reqs/sec         0.15       2.77      52.57
+  Latency        40.35s     15.50s      1.29m
+  Latency Distribution
+     50%     38.35s
+     75%     45.28s
+     90%      0.99m
+     95%      1.15m
+     99%      1.29m
+  HTTP codes:
+    1xx - 0, 2xx - 19, 3xx - 0, 4xx - 0, 5xx - 0
+    others - 0
+  Throughput:    20.11KB/s
+```
+
+Since the upstream bandwidth is limited to 10KB/s and the downstream bandwidth is not restricted, bombardier reported an overall bandwidth of 20.11KB/s.

site/content/zh-hans/docs/reference/plugins/bandwidth_limit.md

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+---
+title: Bandwidth Limit
+---
+
+## 说明
+
+`bandwidthLimit` 插件通过利用 Envoy 的 `bandwidth_limit` 过滤器限制数据流的最大带宽。注意该限制只涉及请求体或响应体。
+
+## 属性
+
+|       |         |
+|-------|---------|
+| Type  | Traffic |
+| Order | Outer   |
+
+## 配置
+
+请参阅相应的 [Envoy 文档](https://www.envoyproxy.io/docs/envoy/v1.28.0/configuration/http/http_filters/bandwidth_limit_filter)。
+
+## 用法
+
+假设我们有下面附加到 `localhost:10000` 的 HTTPRoute，并且有一个后端服务器监听端口 `8080`：
+
+```yaml
+apiVersion: gateway.networking.k8s.io/v1
+kind: HTTPRoute
+metadata:
+  name: default
+spec:
+  parentRefs:
+  - name: default
+    namespace: default
+  rules:
+  - matches:
+    - path:
+        type: PathPrefix
+        value: /
+    backendRefs:
+    - name: backend
+      port: 8080
+```
+
+通过应用下面的配置，发送到 `http://localhost:10000/` 的请求的上传速度将被限制在大约 1kb/s：
+
+```yaml
+apiVersion: mosn.io/v1
+kind: HTTPFilterPolicy
+metadata:
+  name: policy
+spec:
+  targetRef:
+    group: gateway.networking.k8s.io
+    kind: HTTPRoute
+    name: default
+  filters:
+    bandwidthLimit:
+      config:
+        statPrefix: policy_bandwidth_limit
+        enableMode: REQUEST
+        limitKbps: 10
+        fillInterval: 0.02s
+```
+
+带宽限制并不是很精准。`fillInterval` 越小，精确度越高。
+
+为了测试插件效果，我们让监听端口 `8080` 的后端服务器返回所有收到的请求。让我们用 [bombardier](https://pkg.go.dev/github.com/codesenberg/bombardier) 试一下：
+
+```
+$ bombardier -m POST -f go.sum -c 10 -t 180s -d 60s -l http://localhost:10000/
+Bombarding http://localhost:10000/ for 1m0s using 10 connection(s)
+[======================================================================================================================================================] 1m0s
+Done!
+Statistics        Avg      Stdev        Max
+  Reqs/sec         0.15       2.77      52.57
+  Latency        40.35s     15.50s      1.29m
+  Latency Distribution
+     50%     38.35s
+     75%     45.28s
+     90%      0.99m
+     95%      1.15m
+     99%      1.29m
+  HTTP codes:
+    1xx - 0, 2xx - 19, 3xx - 0, 4xx - 0, 5xx - 0
+    others - 0
+  Throughput:    20.11KB/s
+```
+
+由于上行带宽被限制在 10KB/s，而下行带宽没有限制，bombardier 报告了整体带宽是 20.11KB/s。