Potential context leak in kafka-clients instrumentation

[trask]

TracingIterator wraps Iterator<ConsumerRecord> returned from kafka, and starts scopes in next() and closes them in hasNext() for each record in the iterator.

This is a clever way to put the context in scope, and may be acceptable, but worth reviewing alternatives, including removing it and only automatically putting context in scope when kafka is used from higher-level APIs, e.g. spring-kafka's MessageListener.onMessage() and vertx-kafka's KafkaReadStream.poll() which takes a handler (we can start a span for the handler and create links to all of the messages in the batch).