[log](mysql command)Log packet when receive COM_SLEEP (apache#47879)

### What problem does this PR solve?

This p2 case test_group_commit_insert_into_lineitem_normal.groovy:111) -
got exception:java.sql.SQLNonTransientConnectionException: Unsupported
command(Sleep)
Don't know why JDBC may send COM_SLEEP to Doris, add log to help
investigate.

fe/fe-core/src/main/java/org/apache/doris/qe/MysqlConnectProcessor.java

@@ -83,7 +83,7 @@ private void handleStmtClose() {
         handleStmtClose(stmtId);
     }
 
-    private void debugPacket() {
+    private String getPacket() {
         byte[] bytes = packetBuf.array();
         StringBuilder printB = new StringBuilder();
         for (byte b : bytes) {
@@ -95,8 +95,12 @@ private void debugPacket() {
             }
             printB.append(" ");
         }
+        return printB.toString().substring(0, 200);
+    }
+
+    private void debugPacket() {
         if (LOG.isDebugEnabled()) {
-            LOG.debug("debug packet {}", printB.toString().substring(0, 200));
+            LOG.debug("debug packet {}", getPacket());
         }
     }
 
@@ -270,6 +274,9 @@ private void dispatch() throws IOException {
             default:
                 ctx.getState().setError(ErrorCode.ERR_UNKNOWN_COM_ERROR, "Unsupported command(" + command + ")");
                 LOG.warn("Unsupported command(" + command + ")");
+                if (command.equals(MysqlCommand.COM_SLEEP)) {
+                    LOG.warn("COM_SLEEP packet: [{}]", getPacket());
+                }
                 break;
         }
     }