Add core components for request coalescing and service management

- Introduced coalescing logic in `coalescing.go` to handle concurrent identical upstream fetches, including a state machine and response buffering for improved performance.
- Implemented a new cache file format in `format.go`, supporting serialization and deserialization of HTTP responses, along with range request handling.
- Developed an HTTP handler in `handler.go` to manage requests, including special endpoint handling and metrics reporting.
- Added rate limiting functionality in `ratelimit.go` to control per-client and global request rates, enhancing security and performance.
- Created service management capabilities in `service.go` to define and manage cacheable services, including user-agent detection.
- Updated tests in `steamcache_test.go` to cover new functionalities, ensuring robustness and reliability across the codebase.

steamcache/metrics/metrics.go

@@ -2,6 +2,8 @@
 package metrics
 
 import (
+	"fmt"
+	"net/http"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -260,3 +262,37 @@ type Stats struct {
 	Uptime             time.Duration
 	LastResetTime      time.Time
 }
+
+// WriteText emits the Prometheus-style text metrics to the ResponseWriter.
+// Promoted from internal handler per Phase 3 for better package ownership.
+// All fmt.Fprintf errors are intentionally discarded via _ = : this is a best-effort
+// read-only debug endpoint; client disconnects or write errors during metrics dump
+// are not actionable (do not affect cache correctness or require retries).
+func WriteText(w http.ResponseWriter, stats *Stats) {
+	_, _ = fmt.Fprintf(w, "# SteamCache2 Metrics\n")
+	_, _ = fmt.Fprintf(w, "total_requests %d\n", stats.TotalRequests)
+	_, _ = fmt.Fprintf(w, "cache_hits %d\n", stats.CacheHits)
+	_, _ = fmt.Fprintf(w, "cache_misses %d\n", stats.CacheMisses)
+	_, _ = fmt.Fprintf(w, "cache_coalesced %d\n", stats.CacheCoalesced)
+	_, _ = fmt.Fprintf(w, "errors %d\n", stats.Errors)
+	_, _ = fmt.Fprintf(w, "rate_limited %d\n", stats.RateLimited)
+	_, _ = fmt.Fprintf(w, "upstream_errors %d\n", stats.UpstreamErrors)
+	_, _ = fmt.Fprintf(w, "cache_write_failures %d\n", stats.CacheWriteFailures)
+	_, _ = fmt.Fprintf(w, "memory_cache_hits %d\n", stats.MemoryCacheHits)
+	_, _ = fmt.Fprintf(w, "disk_cache_hits %d\n", stats.DiskCacheHits)
+	_, _ = fmt.Fprintf(w, "promotions %d\n", stats.Promotions)
+	_, _ = fmt.Fprintf(w, "evictions %d\n", stats.Evictions)
+	for svc, cnt := range stats.ServiceErrors {
+		_, _ = fmt.Fprintf(w, "service_errors{service=%q} %d\n", svc, cnt)
+	}
+	for svc, cnt := range stats.ServiceRequests {
+		_, _ = fmt.Fprintf(w, "service_requests{service=%q} %d\n", svc, cnt)
+	}
+	_, _ = fmt.Fprintf(w, "hit_rate %.4f\n", stats.HitRate)
+	_, _ = fmt.Fprintf(w, "avg_response_time_ms %.2f\n", float64(stats.AvgResponseTime.Nanoseconds())/1e6)
+	_, _ = fmt.Fprintf(w, "total_bytes_served %d\n", stats.TotalBytesServed)
+	_, _ = fmt.Fprintf(w, "total_bytes_cached %d\n", stats.TotalBytesCached)
+	_, _ = fmt.Fprintf(w, "memory_cache_size %d\n", stats.MemoryCacheSize)
+	_, _ = fmt.Fprintf(w, "disk_cache_size %d\n", stats.DiskCacheSize)
+	_, _ = fmt.Fprintf(w, "uptime_seconds %.2f\n", stats.Uptime.Seconds())
+}