Add Prometheus exporter for cache hit ratio metrics

[Gracora]

Summary

Added a Prometheus metrics exporter binary (peerx-metrics-exporter) that polls the on-chain get_cache_stats entry point and exposes cache hit/miss statistics as scrapeable metrics.

Changes

• bin/peerx-metrics-exporter/: New Rust binary that polls get_cache_stats via Soroban RPC and serves Prometheus metrics on /metrics (port 9090)
• dashboards/cache.json: Grafana dashboard with 4 panels — cache hit ratio (stat), hits vs misses (time series), poll errors (stat), last poll timestamp (stat)
• docs/OPERATIONS.md: Systemd unit file, Prometheus scrape config, build instructions, Docker alternative
• Cargo.toml: Added bin/peerx-metrics-exporter to workspace members
• No unrelated changes introduced

Closes #93

[ameeribro4-sudo]

LGTM