fix(metrics): remove duplicate replication lag migration keys

Signed-off-by: Diana Zawadzki <dzawa@live.de>

Author: zawadzkidiana

common/metrics/config.go

@@ -49,8 +49,6 @@ var HistogramMigrationMetrics = map[string]struct{}{
 
 	// Replication tasks lag/returned/diff (replication task-ack path).
 	// Dual-emitted as timer + histogram.
-	"replication_tasks_lag":                  {},
-	"replication_tasks_lag_ns":               {},
 	"replication_tasks_returned":             {},
 	"replication_tasks_returned_counts":      {},
 	"replication_tasks_returned_diff":        {},