Add exportable to Span and use it to determine if data are exported

If the flag is set then annotations are collected and the data are exported
in zipkin or stream. If not you still get the correlation ids, so a purely
log-oriented solution will always have useful data on all requests.

spring-cloud-sleuth-stream/src/main/java/org/springframework/cloud/sleuth/stream/StreamSpanListener.java

@@ -95,7 +95,9 @@ public class StreamSpanListener {
 	@Order(0)
 	public void release(SpanReleasedEvent event) {
 		event.getSpan().addTimelineAnnotation("release");
-		this.queue.add(event.getSpan());
+		if (event.getSpan().isExportable()) {
+			this.queue.add(event.getSpan());
+		}
 	}
 
 	@InboundChannelAdapter(value = SleuthSource.OUTPUT)