Implement metrics subsystem #36
Description
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