It would be convenient to expose a Peek function for Cursor, so that, if one is interested in allocating a byte slice to accommodate the Decode(), they can do it. Otherwise, we have to guess-timate the size of the byte slice to pass to Decode, or look for NewErrTooSmall errors, which is not optimal.

relevant code: https://github.com/mongodb/mongo-go-driver/blob/2bc72ab2a7bf23f6f53554e9cf48b061943d664e/bson/decode.go#L181-L197