I think you could also use IFTTT, because it is much easier.
Your webhook would just have to look like:
{
"eventName": "to_google_sheets",
"url": "https://maker.ifttt.com/trigger/to_google_sheets/with/key/PUT_YOUR_IFTTT_KEY_HERE",
"requestType": "POST",
"json": {
"value1": "{{SPARK_EVENT_VALUE}}"
},
"mydevices": true
}
Replace PUT_YOUR_IFTTT_KEY_HERE
with your key from the IFTTT Maker Channel.
To log data, you would just make your Photon do:
Particle.publish("to_google_sheets", "Data_you_want_to_Publish");