foo.py

import json

newdata = []

with open("data.json", "r", encoding="utf8") as file:
  data = json.load(file)
  for elem in data:
    newdata.append(
      f"Convert the following message to json format: {elem['message']}\n"+
      f"Details: {elem['labels']}\n"
    )

# write modified data to data3.json
with open("data3.json", "w", encoding="utf8") as file:
  json.dump(newdata, file, indent=4)