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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El issue no menciona archivos, pruebas ni puntos de entrada. Comienza localizando las interfaces de IoTDB para la gestión y la monitorización de conexiones; después, aclara cómo deben exponerse los recuentos de conexiones del usuario actual y totales, y verifica el comportamiento con pruebas específicas.

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