ops: Signal disk-tier attach pending vs ready (#45)

disk_tier_ready + X-SteamCache-Disk-Tier. Closes #33.

steamcache/metrics/metrics.go

@@ -33,6 +33,7 @@ type Metrics struct {
 	DiskCacheHits   int64
 	Promotions      int64
 	Evictions       int64
+	DiskTierReady   int64 // 0=pending (or unset), 1=ready or no-disk (N/A)
 
 	// Expanded observability (upstream breakdowns, cache write failures, per-service errors)
 	UpstreamErrors     int64
@@ -127,6 +128,17 @@ func (m *Metrics) SetDiskCacheSize(size int64) {
 	atomic.StoreInt64(&m.DiskCacheSize, size)
 }
 
+// SetDiskTierReady sets whether the disk slow tier is attached (1) or still pending (0).
+// Memory-only (no disk) also uses 1 — meaning "not waiting on disk attach". Reset does not clear this.
+func (m *Metrics) SetDiskTierReady(ready int64) {
+	atomic.StoreInt64(&m.DiskTierReady, ready)
+}
+
+// GetDiskTierReady returns 1 if disk tier is ready (or no disk configured), else 0 while attach pending.
+func (m *Metrics) GetDiskTierReady() int64 {
+	return atomic.LoadInt64(&m.DiskTierReady)
+}
+
 // IncrementMemoryCacheHits increments memory cache hits
 func (m *Metrics) IncrementMemoryCacheHits() {
 	atomic.AddInt64(&m.MemoryCacheHits, 1)
@@ -211,6 +223,7 @@ func (m *Metrics) GetStats() *Stats {
 		TotalBytesSaved:    atomic.LoadInt64(&m.TotalBytesSaved),
 		MemoryCacheSize:    atomic.LoadInt64(&m.MemoryCacheSize),
 		DiskCacheSize:      atomic.LoadInt64(&m.DiskCacheSize),
+		DiskTierReady:      atomic.LoadInt64(&m.DiskTierReady),
 		MemoryCacheHits:    atomic.LoadInt64(&m.MemoryCacheHits),
 		DiskCacheHits:      atomic.LoadInt64(&m.DiskCacheHits),
 		Promotions:         atomic.LoadInt64(&m.Promotions),
@@ -272,6 +285,7 @@ type Stats struct {
 	MemoryCacheSize  int64
 
 	DiskCacheSize      int64
+	DiskTierReady      int64
 	MemoryCacheHits    int64
 	DiskCacheHits      int64
 	Promotions         int64
@@ -318,5 +332,6 @@ func WriteText(w http.ResponseWriter, stats *Stats) {
 
 	_, _ = fmt.Fprintf(w, "memory_cache_size %d\n", stats.MemoryCacheSize)
 	_, _ = fmt.Fprintf(w, "disk_cache_size %d\n", stats.DiskCacheSize)
+	_, _ = fmt.Fprintf(w, "disk_tier_ready %d\n", stats.DiskTierReady)
 	_, _ = fmt.Fprintf(w, "uptime_seconds %.2f\n", stats.Uptime.Seconds())
 }