TSDB: Lock round access to labels, where necessary

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

tsdb/ooo_head_read.go

@@ -347,7 +347,7 @@ func (ir *OOOCompactionHeadIndexReader) Series(ref storage.SeriesRef, builder *l
 		ir.ch.head.metrics.seriesNotFound.Inc()
 		return storage.ErrNotFound
 	}
-	builder.Assign(s.lset)
+	builder.Assign(s.labels())
 
 	s.Lock()
 	defer s.Unlock()
@@ -494,7 +494,7 @@ func (oh *HeadAndOOOIndexReader) Series(ref storage.SeriesRef, builder *labels.S
 		oh.head.metrics.seriesNotFound.Inc()
 		return storage.ErrNotFound
 	}
-	builder.Assign(s.lset)
+	builder.Assign(s.labels())
 
 	if chks == nil {
 		return nil