feat(metrics): add 20 SRE-focused Prometheus metrics across all modules

[p2pdkivenko]

Summary

• Add 20 new Prometheus metrics across all modules for comprehensive SRE observability
• Instrument every layer: HTTP proxy, resolver, singleflight, cache, discovery, circuit breaker, Redis backend
• Refactor metrics test module with shared test_metrics() helper to eliminate duplication

New Metrics

Metric	Type	Module	Purpose
request_duration_seconds	HistogramVec	proxy	Overall HTTP latency by method/type/status
response_bytes_total	CounterVec	proxy	Response size tracking from Content-Length
rate_limit_rejected_total	Counter	main	Rate limiter rejections
tags_resolve_duration_seconds	HistogramVec	proxy	Tag list resolution latency
negative_cache_hits_total	Counter	resolver	Image-not-found served from cache
cache_stale_serves_total	Counter	resolver	Stale entries served during revalidation
singleflight_inflight	Gauge	resolver	Active singleflight groups
singleflight_wait_duration_seconds	Histogram	resolver	Follower wait time
upstream_connection_errors_total	CounterVec	resolver	Connection failures by reason (timeout/connect/other)
blob_probe_total	CounterVec	proxy	HEAD probe outcomes (found/not_found)
fanout_size	Histogram	resolver	Projects per parallel fanout
discovery_errors_total	Counter	discovery	Refresh failures
discovery_last_success_timestamp_seconds	Gauge	discovery	Epoch of last successful refresh
discovery_duration_seconds	Histogram	discovery	Refresh cycle duration
circuit_breaker_transitions_total	CounterVec	circuit_breaker	State transitions (from→to per project)
redis_operations_total	CounterVec	cache	Redis ops by type and result
redis_fallback_total	Counter	cache	Local Moka fallback activations
redis_reconnections_total	Counter	cache	Sentinel reconnection attempts
cache_entries	GaugeVec	resolver	Current local cache size
build_info	GaugeVec	metrics	Version + commit metadata

Changes by file

• src/metrics.rs — All 20 metric definitions in struct + registrations. Test refactor with test_metrics() helper.
• src/cache.rs — entry_count() on CacheBackend trait. Redis operation/fallback/reconnection counters.
• src/discovery.rs — Timing, error counting, last-success timestamp in refresh().
• src/resolver.rs — Singleflight inflight/wait, negative cache hits, stale serves, fanout size, upstream connection errors, cache entry gauge.
• src/proxy.rs — Request duration, response bytes, tags resolve duration, blob probe outcomes.
• src/circuit_breaker.rs — state_name() helper + transition counter on all CAS-successful state changes.
• src/main.rs — Rate limit rejection counter.

Verification

• cargo check --all-features ✅
• cargo clippy --all-features -- -D warnings ✅ zero warnings
• cargo test --all-features ✅ 108/108 pass
• cargo fmt --check ✅