Shortcut callback execution if there are 1 or fewer elements (#11446)

**What this PR does / why we need it**:
In the case where we have zero elements, we can exit early; with 1
element there's no need for concurrency.
This will provide a small optimisation and also make our
profiles/execution traces easier to read.

We have a fairly significant amount of requests which don't require
concurrency.

pkg/storage/stores/shipper/indexshipper/downloads/index_set.go

@@ -209,15 +209,28 @@ func (t *indexSet) ForEachConcurrent(ctx context.Context, callback index.ForEach
 	}
 	defer t.indexMtx.rUnlock()
 
+	logger := util_log.WithContext(ctx, t.logger)
+	level.Debug(logger).Log("index-files-count", len(t.index))
+
+	if len(t.index) == 0 {
+		return nil
+	}
+
+	// shortcut; if there's only one index, there's no need for bounded concurrency
+	if len(t.index) == 1 {
+		for i := range t.index {
+			idx := t.index[i]
+			return callback(t.userID == "", idx)
+		}
+	}
+
+	//nolint:ineffassign,staticcheck
 	g, ctx := errgroup.WithContext(ctx)
 	if t.maxConcurrent == 0 {
 		panic("maxConcurrent cannot be 0, indexSet is being initialized without setting maxConcurrent")
 	}
 	g.SetLimit(t.maxConcurrent)
 
-	logger := util_log.WithContext(ctx, t.logger)
-	level.Debug(logger).Log("index-files-count", len(t.index))
-
 	for i := range t.index {
 		idx := t.index[i]
 		g.Go(func() error {

pkg/storage/stores/shipper/indexshipper/tsdb/multi_file_index.go

@@ -35,6 +35,15 @@ type IndexIter interface {
 type IndexSlice []Index
 
 func (xs IndexSlice) For(ctx context.Context, maxConcurrent int, fn func(context.Context, Index) error) error {
+	if len(xs) == 0 {
+		return nil
+	}
+
+	// shortcut; if there's only one slice, there's no need for bounded concurrency
+	if len(xs) == 1 {
+		return fn(ctx, xs[0])
+	}
+
 	g, ctx := errgroup.WithContext(ctx)
 	if maxConcurrent == 0 {
 		panic("maxConcurrent cannot be 0, IndexIter is being called with a maxConcurrent of 0")