feat: expose metering data via tenant API

[dennisonbertram]

Problem

CPU, memory, and network usage data is already collected in metering.db (tables: usage_events, usage_daily) but there are no API endpoints for tenants to access it. Agents need observability of their resource consumption for cost optimization, quota planning, and auto-scaling decisions.

Proposed Solution

Add two endpoints:

GET /v1/tenant/usage/metrics

Returns aggregated resource usage for the current tenant.

Query parameters:

• period — hourly | daily (default: daily)
• since — RFC3339 timestamp (default: 24h ago)
• until — RFC3339 timestamp (default: now)
• service_id — optional filter to a specific service

Response:

{
  "metrics": [
    {
      "timestamp": "2026-03-22T00:00:00Z",
      "cpu_seconds": 3600.5,
      "memory_mb_avg": 256.3,
      "network_rx_bytes": 104857600,
      "network_tx_bytes": 52428800,
      "service_id": "svc-abc123"
    }
  ],
  "period": "daily",
  "tenant_id": "tenant-xyz"
}

GET /v1/services/{serviceID}/metrics

Returns resource usage for a specific service. Same query params minus service_id.

Implementation Notes

• Read from existing metering.db tables — no new data collection needed
• Add a MeteringStore that wraps the metering SQLite connection
• Wire into ServerConfig alongside existing Store
• Respect existing auth middleware (tenant can only see own data)
• Add pagination (limit/offset) if result sets could be large

Acceptance Criteria

• GET /v1/tenant/usage/metrics returns aggregated usage data
• GET /v1/services/{id}/metrics returns per-service usage data
• period, since, until query params work correctly
• Tenant isolation enforced (can't see other tenants' data)
• Unit tests for the metering store and handlers
• API reference updated

[dennisonbertram]

PR created: #143