Implement metrics subsystem

[copyleftdev]

Depends on: #18

Implement src/metrics.zig:

• Prometheus-style endpoint
• Counters: messages committed, view changes
• Histograms: latency (prepare, commit, view change)
• Gauges: current view, commit_num, queue depths

Acceptance:

• Metrics exposed on HTTP
• All key metrics tracked
• Prometheus can scrape