GH-3104 Add JsonGetter to show SpEL expression as String

This is specifically done for actuator binding endpoint to avoid type conversion

Resolves #3104

binders/kafka-binder/spring-cloud-stream-binder-kafka-core/src/main/java/org/springframework/cloud/stream/binder/kafka/properties/KafkaProducerProperties.java

@@ -23,6 +23,9 @@ import jakarta.validation.constraints.NotNull;
 
 import org.springframework.expression.Expression;
 
+import com.fasterxml.jackson.annotation.JsonGetter;
+import com.fasterxml.jackson.annotation.JsonValue;
+
 /**
  * Extended producer properties for Kafka binder.
  *
@@ -187,6 +190,14 @@ public class KafkaProducerProperties {
 	public Expression getMessageKeyExpression() {
 		return this.messageKeyExpression;
 	}
+	
+	@JsonGetter("messageKeyExpression")
+	public String getTheMessageKeyExpression() {
+		if (this.messageKeyExpression != null) {
+			return this.messageKeyExpression.getExpressionString();
+		}
+		return null;
+	}
 
 	public void setMessageKeyExpression(Expression messageKeyExpression) {
 		this.messageKeyExpression = messageKeyExpression;