chore: add replication task latency histogram

Add exponential histogram for replication task generation latency in
TaskAckManager, alongside the existing timer.

- Dual-emit ExponentialTaskLatency (task_latency_ns) histogram with
  the existing TaskLatency timer in TaskAckManager.getTasks()
- Register task_latency and task_latency_ns in
  HistogramMigrationMetrics allowlist for configurable rollout

CadenceCDNC-17610

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": {},
 

common/metrics/defs.go

@@ -2480,6 +2480,7 @@ const (
 const (
 	TaskRequests = iota + NumCommonMetrics
 	TaskLatency
+	ExponentialTaskLatency
 	TaskFailures
 	TaskDiscarded
 	TaskAttemptTimer
@@ -3309,6 +3310,7 @@ var MetricDefs = map[ServiceIdx]map[MetricIdx]metricDefinition{
 	History: {
 		TaskRequests:                     {metricName: "task_requests", metricType: Counter},
 		TaskLatency:                      {metricName: "task_latency", metricType: Timer},
+		ExponentialTaskLatency:           {metricName: "task_latency_ns", metricType: Histogram, exponentialBuckets: Low1ms100s},
 		TaskAttemptTimer:                 {metricName: "task_attempt", metricType: Timer},
 		TaskFailures:                     {metricName: "task_errors", metricType: Counter},
 		TaskDiscarded:                    {metricName: "task_errors_discarded", metricType: Counter},

service/history/replication/task_ack_manager.go

@@ -131,8 +131,13 @@ func (t *TaskAckManager) getTasks(ctx context.Context, pollingCluster string, la
 		lastReadTaskID = previousReadTaskID
 	}
 
+	// Keep timer (backwards compatible), dual-emit exponential histogram for migration.
+	taskGeneratedStart := t.timeSource.Now()
 	taskGeneratedTimer := t.scope.StartTimer(metrics.TaskLatency)
 	defer taskGeneratedTimer.Stop()
+	defer func() {
+		t.scope.ExponentialHistogram(metrics.ExponentialTaskLatency, t.timeSource.Since(taskGeneratedStart))
+	}()
 
 	batchSize := t.dynamicTaskBatchSizer.value()
 	t.scope.UpdateGauge(metrics.ReplicationTasksBatchSize, float64(batchSize))