Add error log when dropping spans
diff --git a/htrace-zipkin/src/main/java/org/cloudera/htrace/impl/ZipkinSpanReceiver.java b/htrace-zipkin/src/main/java/org/cloudera/htrace/impl/ZipkinSpanReceiver.java
index f92491b..67a6ba1 100644
--- a/htrace-zipkin/src/main/java/org/cloudera/htrace/impl/ZipkinSpanReceiver.java
+++ b/htrace-zipkin/src/main/java/org/cloudera/htrace/impl/ZipkinSpanReceiver.java
@@ -281,7 +281,7 @@
             try {
               queue.addAll(dequeuedSpans);
             } catch (IllegalStateException ex) {
-              // Just drop them if queue is full.
+              LOG.error("Drop " + dequeuedSpans.size() + " span(s) because queue is full");
             }
           }