metrics: allowlist task_latency migration

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

Author: zawadzkidiana

common/metrics/config.go

@@ -41,6 +41,11 @@ func (h *HistogramMigration) UnmarshalYAML(read func(any) error) error {
 // This is likely best done in an `init` func, to ensure it happens early enough
 // and does not race with config reading.
 var HistogramMigrationMetrics = map[string]struct{}{
+	// History task generation latency (replication task-ack path).
+	// Dual-emitted as timer + histogram.
+	"task_latency":    {},
+	"task_latency_ns": {},
+
 	"task_latency_processing":    {},
 	"task_latency_processing_ns": {},