Stream reactive (#2038)

Added native support for Reactive Spring Cloud Stream.

spring-cloud-sleuth-api/src/main/java/org/springframework/cloud/sleuth/Tracer.java

@@ -140,6 +140,14 @@ public interface Tracer extends BaggageManager {
 	 */
 	TraceContext.Builder traceContextBuilder();
 
+	/**
+	 * Returns the {@link CurrentTraceContext}. Can be {@code null} so that we don't break
+	 * backward compatibility.
+	 * @return current trace context
+	 */
+	@Nullable
+	CurrentTraceContext currentTraceContext();
+
 	/**
 	 * Allows to customize the current span in scope.
 	 * @return current span customizer