mongo: how should I layout my data
Yes, create a new record for every reading.

MongoDB and other document stores are designed for efficient retrieval of de-normalized or irregularly structured data. A MongoDB collection is essentially a set of key-value pairs with a rich value type. Updating the value is an atomic operation, which means that constantly appending to a list within the value is not going to be efficient and, as you mentioned, you will eventually hit the document size limit.