Reading Json object name in arduino
There is a brilliant JSON library called ArduinoJSON. It can be installed using the arduino library manager.

See https://bblanchon.github.io/ArduinoJson/doc/decoding/ for further details.

To loop through the keys, you can use:
https://bblanchon.github.io/ArduinoJson/api/jsonobject/begin_end/

or if you know the key, you can do jsonObject["pi"] to get the value