Assuming you're on 2013.1+:

set data = "{""profile_id"":""9XOzzcI8NfSUjxAhEt0cTLRejwmp6HPi"",""biometrics"":[{""timestamp"":""2017-05-17T13:45:40"",""utc_offset"":""+02:00"",""resting_heartrate"":120.0,""spo2"":98.0,""activity_id"":""591c540aac8f295479ee14ce""}]}"

do ##class(%ZEN.Auxiliary.jsonProvider).%ParseJSON(data,,.object)
write object."profile_id"
write object.biometrics.GetAt(1).timestamp