[Enhancement] decrease FE warn log when client use conn pool and close channel

[CHzxp]

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Description

In many scenarios, clients use connection pools to access Doris . By tcp dump, it is found that each client sends sql to the service with a temporary network port. After sending sql, it will send a FIN message which size is 0 to FE server,when the server recive the FIN message, FE think that the client has closed the channel and print lots of warn logs. However, if a large number of clients frequently send sql and send FIN, FE will print a large number of warn logs. I think this is unreasonable, because this is a normal client disconnection, and printing a large number of warn logs will cause Let users doubt whether the service status is normal, so I suggest lowering the warn log level, because I think it is a normal phenomenon for the client to close the channel, and there should be no large number of alarms.

Solution

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[CHzxp]

this is the tcp dump content

[CHzxp]

this is the fe log