Kafka sender: Use metadata to pick topic

[KristianLyng]

Similar to the influxdb sender, a "TopicFromMeatadata" would reduce the needed configuration significantly.

At the moment, I need to specify 8 senders to send to 8 different topics, then set up a switch sender to determine what goes where. This is a bit cumbersome.

However, the Kafka-go library requires that a Writer object specifies the topic to send to, and reasonably disallows modifying the Writer-object if used concurrently. So unlike the influx sender, where we just write the measurement in the line protocol format, we need to dynamically allocate Writer-objects on demand, and possibly have some sort of safety mechanism to avoid arbitrary amounts of Writers.

As such, this is a bit of a future-item since it's a bit of work.