code: func (mc MongoClient) UpdateReading(r contract.Reading) error {
  	var mapped models.Reading
  	id, err := mapped.FromContract(r)
  	if err != nil {
  		return err
  	}
  
  	mapped.TimestampForUpdate()
  
  	return mc.updateId(db.ReadingsCollection, id, mapped)
  }
docs: // Update a reading
  // 404 - reading cannot be found
  // 409 - Value descriptor doesn't exist
  // 503 - unknown issues