Systemd can print logs as JSON, making custom log formats simple. In addition, if the messages themselves are JSON, jq can unescape them.

show-systems-json-messages.markdown

$ journalctl -u concourse-web -f -o json | jq '.MESSAGE | fromjson'

{
  "timestamp": "2024-07-27T17:29:50.525878425Z",
  "level": "info",
  "source": "atc",
  "message": "atc.scanner.tick.start",
  "data": {
    "session": "23.37"
  }
}
{
  "timestamp": "2024-07-27T17:29:50.542058050Z",
  "level": "info",
  "source": "atc",
  "message": "atc.scanner.tick.end",
  "data": {
    "session": "23.37"
  }
}