[Task] Custom Prometheus collector and /metrics endpoint

[arasHi87]

Description

Emit cumulative billing counters from GlobalState via a custom prometheus.Collector (not CounterVec, which resets on restart → negative rate()), served on /metrics.

Acceptance Criteria

A detailed list of conditions that must be met for the story to be accepted. These statements must focus on functional behavior and constraints rather than specific code-level implementation.

• Custom Collector holds RLock across the GlobalState walk
• cubecos_bytes_total/_packets_total {tenant_id,zone,direction}
• Zero-alloc Collect (bench-gated)

Story Points: 3

Points	Effort / Resources	Description
1	Low	Low complexity; can be resolved easily.
3	Medium	Moderate workload; requires some focus, but risks are well-managed.
5	High	High complexity; difficult to implement, requiring significant time and effort.
8	Very High	Extreme complexity or high uncertainty. If possible, this should be divided into multiple user stories.

Notes

Delivered in PR #8. Sprint 2 (2026-05-15).