* [TEST] Add mock_monitor test helper for metrics testing

Add thread-safe mock implementation of monitoring_interface for testing.
Features:
- Records all reported metrics (counters, gauges, histograms, health)
- Thread-safe with atomic counters and mutex protection
- Helper methods for test assertions (has_counter, get_counter_value)
- Support for verifying metric labels and values

Part of #405

* [TEST] Add network metrics unit tests

Add comprehensive unit test coverage for metric_reporter and
monitoring_integration classes.

Test categories:
- Metric name constants (5 tests)
- Reporter methods: connection, bytes, latency, errors (9 tests)
- Edge cases: zero, large values, empty strings (6 tests)
- Thread safety: concurrent counter/mixed reporting (2 tests)
- Accumulation and gauge behavior (2 tests)
- MonitoringIntegrationManager singleton (3 tests)
- BasicMonitoring implementation (4 tests)

Total: 31 unit tests covering all public interfaces.

Part of #405

* [TEST] Add network metrics integration tests

Add integration tests validating real-world metrics usage patterns.

Test scenarios:
- Connection lifecycle metrics flow
- Error handling metrics (connection_failed, errors, timeouts)
- Session metrics with duration tracking
- High-volume data transfer (100 packets, MTU size)
- Latency distribution recording
- Direct manager reporting
- Health reporting verification
- Concurrent connection tracking (4 threads, 25 connections each)
- Default monitoring fallback behavior
- Custom monitoring implementation pluggability

Total: 10 integration tests covering metrics integration.

Part of #405

* docs: update CHANGELOG for metrics test coverage

Document the addition of network metrics test coverage in both
English and Korean changelogs.

Relates to #405