in iot_code/DemoProject/src/main.cpp [60:69]
void publishMessage()
{
StaticJsonDocument<200> doc;
doc["time"] = millis();
doc["sensor_a0"] = analogRead(0);
char jsonBuffer[512];
serializeJson(doc, jsonBuffer); // print to client
client.publish(AWS_IOT_PUBLISH_TOPIC, jsonBuffer);
}