For efficiency we're using the Kafka clients send_messages function which takes
a variable number of messages to send and batches them so that they can be read
one at a time. To do this we use the * operator to expand the list of metrics
as we pass them to the send_messages function. If the input data is not a list
this doesn't work so even single metrics need to be bundled in a list.
Change-Id: I2f196b37db0db95a5163696f14258a865fa9a615
08c200b6a8