ops: Disk tier occupancy on /metrics

Operators could see attach-ready and capacity-pressure events but not how
full the configured disk (or memory) tier was without reading filesystems.
Expose size/capacity gauges and disk_cache_full_ratio next to disk_tier_ready.
Capacity is a config read, so it stays available while attach is pending.

steamcache/steamcache.go

@@ -352,6 +352,11 @@ func (sc *SteamCache) Shutdown() {
 func (sc *SteamCache) GetMetrics() *metrics.Stats {
 	if sc.memory != nil {
 		sc.metrics.SetMemoryCacheSize(sc.memory.Size())
+		sc.metrics.SetMemoryCacheCapacity(sc.memory.Capacity())
+	}
+	if sc.disk != nil {
+		// Capacity() is a plain config field — safe to read even while disk attach is pending.
+		sc.metrics.SetDiskCacheCapacity(sc.disk.Capacity())
 	}
 	// Skip disk.Size() while attach pending — Size() blocks on initDone and would hang /metrics.
 	if sc.disk != nil && sc.metrics.GetDiskTierReady() == 1 {