feat(metis): add Prometheus metrics collection and HTTP endpoint

[YifeiZhuang]

This change introduces Prometheus metrics for the Metis daemon and CNI plugin to monitor IP address allocation, capacity utilization, gRPC RPC latencies, and controller sync actions.

Key Features & Changes:

1. Metrics Package (pkg/metrics):

   • Created a standalone pkg/metrics package using promauto. promauto is better for Metis because all metrics in pkg/metrics are static package-level variables initialized once when the process starts. It eliminates repetitive init() boilerplate while guaranteeing that all metrics are registered before any handler or daemon component uses them.
   • Exported metrics for:
     • Daemon gRPC requests (metis_grpc_server_handled_total, metis_rpc_latency_seconds)
     • CNI plugin calls (metis_cni_plugin_handled_total, metis_cni_rpc_latency_seconds)
     • Dynamic allocation requests (metis_outgoing_dynamic_ip_alloc_request_total, metis_dynamic_ip_alloc_rpc_latency_seconds)
     • Local store IP & CIDR capacity/utilization (metis_local_store_ip_total, metis_local_store_cidr_block_total)
     • Watcher & Monitor operations (metis_watcher_cidr_operation_count, metis_monitor_action_count, metis_pending_dynamic_request)
   • Documented all metrics with detailed Go doc comments.

2. Configurable Metrics HTTP Server:
   - Added --metrics-port daemon option (default 9996).
   - If --metrics-port > 0, an HTTP server exposes /metrics using promhttp.Handler(),
   and metrics collection is enabled across all daemon sub-components (Server, Engine, Monitor, Watcher).
   - If --metrics-port <= 0 (or 0), the HTTP server is not started and Prometheus metric recording is disabled.

3. Store IP & CIDR Accounting Refactoring:

   • Refactored NetworkIPUsage into nested IPs (IPUsage) and CIDRs (CIDRUsage) structs
     for clean separation of IP states (allocated, cooldown, draining, deleting, active total, total)
     and CIDR block states (ready, draining, deleting).

cc. @arvindbr8 @gnossen @zhaoqsh

[kubernetes-prow]

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[kubernetes-prow]

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[kubernetes-prow]

PR needs rebase.

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