docs: add instance metrics reference page

[JeromeJaggi]

Summary

Adds a reference page for the instance metrics endpoint.

What's included

• Endpoints (GET by UUID, bulk query, POST body)
• Response format selection via Accept header (JSON vs Prometheus text)
• Full JSON field reference grouped by category: identity, lifecycle, boot timing, resource usage, network throughput, connection activity, wakeup latency histogram
• Prometheus metric name list
• Example curl request and response

Source verification

Content verified against GET /v1/instances/{uuid}/metrics and the Unikraft Cloud REST API reference.

[JeromeJaggi]

@marcrittinghaus