Add missing mergeStats in garbage collector and cache writer threads (#8392)

Co-authored-by: Artyom Abakumov <[email protected]>

Author: Noremos

src/jrd/cch.cpp

@@ -3079,7 +3079,10 @@ void BufferControl::cache_writer(BufferControl* bcb)
 				{
 					BufferDesc* const bdb = get_dirty_buffer(tdbb);
 					if (bdb)
+					{
 						write_buffer(tdbb, bdb, bdb->bdb_page, true, &status_vector, true);
+						attachment->mergeStats();
+					}
 				}
 
 				// If there's more work to do voluntarily ask to be rescheduled.

src/jrd/vio.cpp

@@ -5497,6 +5497,8 @@ void Database::garbage_collector(Database* dbb)
 
 						CCH_flush(tdbb, FLUSH_SWEEP, 0);
 						flush = false;
+
+						attachment->mergeStats();
 					}
 
 					dbb->dbb_flags &= ~DBB_gc_active;