[Studio] feat: Operation and Maintenance Dashboard Metrics Subsystem

[zhaohai666]

Summary

• Add ops-level dashboard metrics subsystem for monitoring broker, topic, consumer group, and system resources
• Introduce MetricsProvider SPI interface with MetricsProviderStub for sample data
• Add REST controller at /api/metrics with overview, broker, topic, and consumer group endpoints
• Add 5 VO classes: BrokerMetricsVO, TopicMetricsVO, ConsumerGroupMetricsVO, MetricsOverviewVO, SystemResourceMetricsVO
• Include test coverage for controller and service layers

Test plan

• Verify MetricsController returns correct overview data
• Verify broker/topic/consumer group metrics endpoints return expected VO structures
• Verify MetricsProviderStub returns valid sample data
• Run MetricsControllerTest and MetricsServiceTest