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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**Additional context**
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该 issue 未指定任何文件、测试或入口点。首先定位 IoTDB 的连接管理和监控接口，然后明确应如何公开当前用户连接数和总连接数，并通过有针对性的测试验证行为。

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