Add bare minimum info to Kafka traces to allow manual debugging

[timmc-edx]

In #758 we're hoping to add distributed tracing for the event bus. However, we may want to take a first step of adding just enough information that we can manually step between producer and consumer traces. This might involve one of the following:

• Adding DD_KAFKA_PROPAGATION_ENABLED on both sides (if it automatically enriches the kafka.produce and kafka.consume spans with enough info)
• Adding message offset info (in a new span, via function_trace) to the producer side.

Neither of these would ensure we end up with both traces actually being sampled and indexed, but they might help in a debugging scenario.

[robrap]

Note: Using function_trace, we can affect span indexing based on the name we give it.