Kafka sink - key not filled in correctly in Version 2.0

[wduczkowski]

Environment:

• eKuiper version: 2.0-full
• Hardware configuration (e.g. lscpu): OS (e.g. cat /etc/os-release): Debian GNU/Linux 11 (bullseye)" Others: deploymen as docker container, image version: docker pull lfedge/ekuiper:2.0-full

The key attribute in the Kafka sink configuration is not correctly populated in eKuiper version 1.14.6+ when using a pattern-based key assignment. The issue was not present in version 1.14.6, where the key was correctly derived from the events generated by the SQL query.

Steps to Reproduce:

1. Kafka Sink Configuration: The following Kafka sink configuration is used:

   
   {
   "kafka": {
       "batchSize": 1,
       "brokers": "192.168.xxx.xx:9092",
       "bufferLength": 1024,
       "enableCache": false,
       "format": "json",
       "insecureSkipVerify": false,
       "key": "{{.key}}",
       "maxAttempts": 1,
       "omitIfEmpty": false,
       "requiredACKs": 1,
       "resourceId": "kafka",
       "runAsync": false,
       "saslAuthType": "none",
       "sendSingle": true,
       "topic": "aggregation.room"
   }
   }

2. Event Stream Data: The SQL query produces events with the following structure:

{ "humidity": 48.2, "key": "TempHumPres", "topic": "TempHumPres/get/sensor/2" }

{ "key": "TempHumPres", "temperature": 22.4, "topic": "TempHumPres/get/sensor/1" }

{ "key": "TempHumPres", "pressure": 1005.7, "topic": "TempHumPres/get/sensor/3" }

 3. Kafka Topic Observation: The Kafka topic receives messages with  key: {{.key}} instead of the expected value derived from the key attribute in the events. Expected Key: TempHumPres

![image](https://github.com/user-attachments/assets/1788777e-c5bf-4f40-a738-8cadec1a4274)

4. Previous Behavior: In version 1.14.6, the same configuration correctly populated the Kafka key based on the key attribute in the event.

![image](https://github.com/user-attachments/assets/4442fed3-e664-4401-8337-beb8d425018b)