Adds notes about default broker name in messaging

spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/instrument/messaging/TracingChannelInterceptor.java

@@ -56,6 +56,20 @@ public final class TracingChannelInterceptor extends ChannelInterceptorAdapter
 		implements ExecutorChannelInterceptor {
 
 	private static final Log log = LogFactory.getLog(TracingChannelInterceptor.class);
+        /**
+         * Using the literal "broker" until we come up with a better solution.
+         *
+         * <p>If the message originated from a binder (consumer binding), there will be different
+         * headers present (e.g. "KafkaHeaders.RECEIVED_TOPIC" Vs. "AmqpHeaders.CONSUMER_QUEUE"
+         * (unless the application removes them before sending). These don't represent the broker,
+         * rather a queue, and in any case the heuristics are not great. At least we might be able
+         * to tell if this is rabbit or not (ex how spring-rabbit works). We need to think this
+         * through before making an api, possibly experimenting.
+         *
+         * <p>If the app is outbound only (producer), there's no indication of what type the
+         * destination broker is. This may hint at a non-manual solution being overwriting the
+         * remoteServiceName later, similar to how servlet instrumentation lazy set "http.route".
+         */
 	private static final String REMOTE_SERVICE_NAME = "broker";
 
 	public static TracingChannelInterceptor create(Tracing tracing) {