[fix](regression)Add log for Reach limit of connections (#45887)

### What problem does this PR solve?

Add log to help investigate Reach limit of connections error.

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

### Release note

None

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

@@ -93,13 +93,17 @@ public boolean submit(ConnectContext context) {
     // Register one connection with its connection id.
     public boolean registerConnection(ConnectContext ctx) {
         if (numberConnection.incrementAndGet() > maxConnections) {
+            LOG.info("Number connection {} has reach upper limit {}. Connection map : [{}]",
+                    numberConnection.get(), maxConnections, connectionMap);
             numberConnection.decrementAndGet();
             return false;
         }
         // Check user
         connByUser.putIfAbsent(ctx.getQualifiedUser(), new AtomicInteger(0));
         AtomicInteger conns = connByUser.get(ctx.getQualifiedUser());
         if (conns.incrementAndGet() > ctx.getEnv().getAuth().getMaxConn(ctx.getQualifiedUser())) {
+            LOG.info("User {}'s connection {} has reached upper limit {}. connByUser: [{}]", ctx.getQualifiedUser(),
+                    conns.get(), ctx.getEnv().getAuth().getMaxConn(ctx.getQualifiedUser()), connByUser);
             conns.decrementAndGet();
             numberConnection.decrementAndGet();
             return false;