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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**Describe alternatives you've considered**
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**Additional context**
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The issue names no files, tests, or entry points. Begin by locating IoTDB’s connection-management and monitoring interfaces, then clarify how current-user and total connection counts should be exposed and verify the behavior with focused tests.

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