Note experimental flag and add comment

Signed-off-by: Nick Pillitteri <nick.pillitteri@grafana.com>

docs/sources/mimir/configure/about-versioning.md

@@ -146,6 +146,7 @@ The following features are currently experimental:
   - Use of Redis cache backend (`-blocks-storage.bucket-store.chunks-cache.backend=redis`, `-blocks-storage.bucket-store.index-cache.backend=redis`, `-blocks-storage.bucket-store.metadata-cache.backend=redis`)
   - `-blocks-storage.bucket-store.series-selection-strategy`
   - Eagerly loading some blocks on startup even when lazy loading is enabled `-blocks-storage.bucket-store.index-header.eager-loading-startup-enabled`
+  - Set a timeout for index-header lazy loading (`-blocks-storage.bucket-store.index-header.lazy-loading-concurrency-queue-timeout`)
 - Read-write deployment mode
 - API endpoints:
   - `/api/v1/user_limits`

pkg/storegateway/bucket_stores.go

@@ -446,6 +446,9 @@ func (t timeoutGate) Start(ctx context.Context) error {
 	defer cancel()
 
 	err := t.delegate.Start(ctx)
+	// Note that we only return an error for a timeout when the delegate has also returned an
+	// error. This ensures that when we get a slot in the delegate, our caller will call Done()
+	// and release the slot.
 	if err != nil && errors.Is(context.Cause(ctx), errGateTimeout) {
 		_ = spanlogger.FromContext(ctx, log.NewNopLogger()).Error(err)
 		err = errGateTimeout