storage: fix TestPebbleMetricEventListener

In #96021 handling of disk slow events was made asynchronous. This introduced a
race during Close where the goroutine logging could still be inflight.

Fix #96635.

Epic: None
Release note: None

pkg/storage/pebble.go

@@ -735,6 +735,7 @@ type Pebble struct {
 		syncutil.Mutex
 		flushCompletedCallback func()
 	}
+	asyncDone sync.WaitGroup
 
 	// supportsRangeKeys is 1 if the database supports range keys. It must
 	// be accessed atomically.
@@ -1006,7 +1007,7 @@ func NewPebble(ctx context.Context, cfg PebbleConfig) (p *Pebble, err error) {
 	oldDiskSlow := lel.DiskSlow
 	lel.DiskSlow = func(info pebble.DiskSlowInfo) {
 		// Run oldDiskSlow asynchronously.
-		go oldDiskSlow(info)
+		p.async(func() { oldDiskSlow(info) })
 	}
 	el := pebble.TeeEventListener(
 		p.makeMetricEtcEventListener(ctx),
@@ -1087,6 +1088,17 @@ func NewPebble(ctx context.Context, cfg PebbleConfig) (p *Pebble, err error) {
 	return p, nil
 }
 
+// async launches the provided function in a new goroutine. It uses a wait group
+// to synchronize with (*Pebble).Close to ensure all launched goroutines have
+// exited before Close returns.
+func (p *Pebble) async(fn func()) {
+	p.asyncDone.Add(1)
+	go func() {
+		defer p.asyncDone.Done()
+		fn()
+	}()
+}
+
 func (p *Pebble) makeMetricEtcEventListener(ctx context.Context) pebble.EventListener {
 	return pebble.EventListener{
 		WriteStallBegin: func(info pebble.WriteStallBeginInfo) {
@@ -1132,7 +1144,7 @@ func (p *Pebble) makeMetricEtcEventListener(ctx context.Context) pebble.EventLis
 
 					log.Fatalf(ctx, "file write stall detected: %s", info)
 				} else {
-					go log.Errorf(ctx, "file write stall detected: %s", info)
+					p.async(func() { log.Errorf(ctx, "file write stall detected: %s", info) })
 				}
 				return
 			}
@@ -1172,6 +1184,9 @@ func (p *Pebble) Close() {
 	}
 	p.closed = true
 
+	// Wait for any asynchronous goroutines to exit.
+	p.asyncDone.Wait()
+
 	handleErr := func(err error) {
 		if err == nil {
 			return