Specify that indexheader.Reader returns LabelNames in order, so we ca…

…n use that.

Signed-off-by: Peter Štibraný <[email protected]>

pkg/block/indexheader/header.go

@@ -35,6 +35,6 @@ type Reader interface {
 	// then empty string is returned and no error.
 	LabelValues(name string) ([]string, error)
 
-	// LabelNames returns all label names.
+	// LabelNames returns all label names in sorted order.
 	LabelNames() ([]string, error)
 }

pkg/store/bucket.go

@@ -1208,13 +1208,12 @@ func (s *BucketStore) LabelNames(ctx context.Context, req *storepb.LabelNamesReq
 			var result []string
 			if len(reqSeriesMatchers) == 0 {
 				// Do it via index reader to have pending reader registered correctly.
+				// LabelNames are already sorted.
 				res, err := indexr.block.indexHeaderReader.LabelNames()
 				if err != nil {
 					return errors.Wrapf(err, "label names for block %s", b.meta.ULID)
 				}
 
-				sort.Strings(res)
-
 				// Add  a set for the external labels as well.
 				// We're not adding them directly to res because there could be duplicates.
 				// b.extLset is already sorted by label name, no need to sort it again.