RavenDB-21936 Fixing the allocation size for a counter group name whe…

…n purging counter tombstones

src/Raven.Server/Documents/CountersStorage.cs

@@ -2055,7 +2055,7 @@ private long PurgeCounters(long upto, DocumentsOperationContext context, Collect
                             table.DeleteByKey(counterGroupKey);
 
                             names.GetPropertyByIndex(1, ref prop);
-                            using var newScope = context.Allocator.Allocate(CounterKeysSlice.Size + 1 /* separator */  + 1 /* replace the current name with next one in counters group */, out ByteString newCounterKey);
+                            using var newScope = context.Allocator.Allocate(documentKeyPrefix.Size /* it includes the separator already */ + prop.Name.Size /* replace the current name with next one in counters group */, out ByteString newCounterKey);
                             documentKeyPrefix.CopyTo(newCounterKey.Ptr);
                             Memory.Copy(newCounterKey.Ptr + documentKeyPrefix.Size, prop.Name.Buffer, prop.Name.Size);
                             Slice.From(context.Allocator, newCounterKey.Ptr, documentKeyPrefix.Size + prop.Name.Size, out counterGroupKey);