Producers: Message keys

- Producers can choose to send a key with the message (string, number, etc..)

- If key = null, data is sent reound robin (broker 101 then 102 then 103...)

- If a key is sent, then all messages for that key will always go to the same partition

- A key is basically sent if you need message ordering for a specific field(ex:truck_id)

     (Advanced: we get this guarantee thanks to key hashing, which depends on the number of partitions)