ops: Signal disk-tier attach pending vs ready

Large disk caches can look memory-only/broken during async DiskFS attach.
Expose disk_tier_ready in /metrics, pending/ready logs for disk-only and
mixed modes, and X-SteamCache-Disk-Tier on /lancache-heartbeat. GetMetrics
skips blocking disk.Size() while attach is pending so /metrics stays usable.

Closes #33

steamcache/steamcache.go

@@ -200,23 +200,33 @@ func New(address string, memorySize string, diskSize string, diskPath, upstream,
 
 	if disksize == 0 && memorysize != 0 {
 		// memory only mode - no disk
+		sc.metrics.SetDiskTierReady(1) // no disk — N/A / not pending
 		c.SetSlow(mgc)
 	} else if disksize != 0 && memorysize == 0 {
 		// disk only mode: delay attach until disk ready (pure-proxy during scan; Create returns ErrNotFound until slow tier Set)
+		sc.metrics.SetDiskTierReady(0)
+		logger.Logger.Info().Msg("Disk slow tier attach pending; Size barrier in progress")
 		sc.wg.Add(1)
 		go func() {
 			defer sc.wg.Done()
+			t0 := time.Now()
 			_ = d.Size() // block on barrier per design (all Size callers during window do this; documented)
 			select {
 			case <-sc.shutdownCh:
 				return // Shutdown raced; do not attach or SetSlow after stop
 			default:
 				c.SetSlow(dgc)
+				sc.metrics.SetDiskTierReady(1)
+				logger.Logger.Info().
+					Dur("attach_delay", time.Since(t0)).
+					Msg("Disk slow tier attached (disk-only mode); prior traffic had no disk tier")
 			}
 		}()
 	} else if disksize != 0 && memorysize != 0 {
 		// memory and disk mode: fast mem immediate, disk delayed (mem-only during scan)
 		c.SetFast(mgc)
+		sc.metrics.SetDiskTierReady(0)
+		logger.Logger.Info().Msg("Disk slow tier attach pending; Size barrier in progress")
 		sc.wg.Add(1)
 		go func() {
 			defer sc.wg.Done()
@@ -227,6 +237,7 @@ func New(address string, memorySize string, diskSize string, diskPath, upstream,
 				return
 			default:
 				c.SetSlow(dgc)
+				sc.metrics.SetDiskTierReady(1)
 				logger.Logger.Info().
 					Dur("attach_delay", time.Since(t0)).
 					Msg("Disk slow tier attached (mixed mode); prior traffic was memory-only")
@@ -326,15 +337,13 @@ func (sc *SteamCache) Shutdown() {
 
 // GetMetrics returns current metrics
 func (sc *SteamCache) GetMetrics() *metrics.Stats {
-	// Update cache sizes
 	if sc.memory != nil {
 		sc.metrics.SetMemoryCacheSize(sc.memory.Size())
 	}
-	if sc.disk != nil {
-		// Note: blocks on initDone (post-eviction state) for accurate post-attach size during long disk init window.
+	// Skip disk.Size() while attach pending — Size() blocks on initDone and would hang /metrics.
+	if sc.disk != nil && sc.metrics.GetDiskTierReady() == 1 {
 		sc.metrics.SetDiskCacheSize(sc.disk.Size())
 	}
-
 	return sc.metrics.GetStats()
 }