connection numbers statistics of IoTDB

**Is your feature request related to a problem? Please describe.**
I want to monitor the connection numbers occupied by current users and the total connection numbers of IoTDB, just like the MySQL. Currently, IoTDB doesn't provide this function. I hope it can provide this useful function in later version.

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La issue non indica file, test o punti di ingresso. Inizia individuando le interfacce di gestione e monitoraggio delle connessioni di IoTDB, quindi chiarisci come devono essere esposti i conteggi delle connessioni dell’utente corrente e totali e verifica il comportamento con test mirati.

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