feat: add mqtt properties in meta (#4010)

Signed-off-by: Song Gao <disxiaofei@163.com>

Author: Yisaer

docs/en_US/guide/sources/builtin/mqtt.md

@@ -188,6 +188,16 @@ For those who prefer a hands-on approach, the Command Line Interface (CLI) provi
 
 More details can be found at [Streams Management with CLI](../../../api/cli/streams.md).
 
+## Message Properties (MQTT v5)
+
+When `protocolVersion` is set to `5`, MQTT v5 `User Properties` from incoming messages will be added into the message metadata (`meta`) under the key `properties` as a `map[string]string`.
+
+Example:
+
+```sql
+SELECT meta(properties) AS props FROM demo
+```
+
 ## Migration Guide
 
 Starting from version 1.5.0, eKuiper has modified the MQTT source broker configuration, transitioning from `servers` to `server`. As a result, users can now specify only a single MQTT broker address, as opposed to an array of addresses.

docs/zh_CN/guide/sources/builtin/mqtt.md

@@ -181,6 +181,16 @@ REST API 为 eKuiper 提供了一种可编程的交互方式，适用于自动
 
 详细操作步骤及命令解释，可参考 [通过 CLI 进行流管理](../../../api/cli/streams.md)。
 
+## 消息属性（MQTT v5）
+
+当 `protocolVersion` 设置为 `5` 时，MQTT v5 的 `User Properties` 会被写入消息元数据 `meta` 中，key 为 `properties`，类型为 `map[string]string`。
+
+**示例**
+
+```sql
+SELECT meta(properties) AS props FROM demo
+```
+
 ## 迁移指南
 
 从 eKuiper 1.5.0 开始，eKuiper 将 MQTT 源地址配置从 `servers` 更改为 `server`，即用户只能配置一个 MQTT 源地址而不是一个地址数组。对需要进行版本升级的用户：

internal/io/mqtt/source.go

@@ -123,6 +123,7 @@ func (ms *SourceConnector) onMessage(ctx api.StreamContext, msg any, ingest api.
 		if tid, ok := props["traceparent"]; ok {
 			meta["traceId"] = tid
 		}
+		meta["properties"] = props
 	}
 	ingest(ctx, payload, meta, rcvTime)
 }