[Feat] Support metrics across language boundary

[flesher0813]

Purpose

What this PR does / why we need it?

This pull request (PR) supports collecting different statistics aggregated by the monitor component from both the Python side (e.g., ucm_connector) and the C++ side (e.g., various stores), and provides a unified interface for users to retrieve such statistics.
Therefore, this PR modifies the current metrics structure in three aspects:

• Implements the Python-side statistics and decouples the implementation from the framework structure.
• Exposes the interfaces required for registering, creating, updating, retrieving, and clearing statistics via global functions, and exposes these interfaces to Python through pybind11 to ensure that both languages can access the same instance within a single process.
• Provides the examples and documentation.

Modifications

Does this PR introduce any user-facing change?

Users can add or remove the statics to be collected according to the documentation for custom metrics.

Test

How was this patch tested?

Tested with

ucm/shared/test/case/metrics/monitor_test.cc
ucm/shared/test/example/metrics/monitor_stats_example.py

doc