A software-as-a-service (SaaS) company specializing in automobile IoT sensors collects streaming time-series data from tens of thousands of vehicles. The vehicles are owned and operated by 40 different companies, who are the primary customers of the SaaS company. The data will be stored in Bigtable using a multitenant database; that is, all customer data will be stored in the same database. The data sent from the IoT device includes a sensor ID, which is globally unique; a timestamp; and several metrics about engine efficiency. Each customer will query their own data only. Which of the following would you use as a row-key?