HADOOP-18082.Add debug log when RPC#Reader gets a Call. (#3891). Contributed by JiangHua Zhu.

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/Server.java

@@ -3029,6 +3029,9 @@ private void internalQueueCall(Call call, boolean blocking)
       } else {
         callQueue.add(call);
       }
+
+      LOG.debug("Call has entered the CallQueue and is waiting to be processed. " +
+          "Call details: {}", call);
       long deltaNanos = Time.monotonicNowNanos() - call.timestampNanos;
       call.getProcessingDetails().set(Timing.ENQUEUE, deltaNanos,
           TimeUnit.NANOSECONDS);