Use processing-latency histogram for task-ack path

Address review feedback by removing task_latency_ns histogram and using
ExponentialTaskProcessingLatency in task_ack_manager. Also remove stale
histogram-migration allowlist entries for task_latency/task_latency_ns.

Signed-off-by: zawadzki <zawadzki@uber.com>

Author: dianazawadzki

common/metrics/config.go

@@ -41,11 +41,6 @@ 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

@@ -2480,7 +2480,6 @@ const (
 const (
 	TaskRequests = iota + NumCommonMetrics
 	TaskLatency
-	ExponentialTaskLatency
 	TaskFailures
 	TaskDiscarded
 	TaskAttemptTimer
@@ -3314,7 +3313,6 @@ 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

@@ -136,7 +136,7 @@ func (t *TaskAckManager) getTasks(ctx context.Context, pollingCluster string, la
 	taskGeneratedTimer := t.scope.StartTimer(metrics.TaskLatency)
 	defer taskGeneratedTimer.Stop()
 	defer func() {
-		t.scope.ExponentialHistogram(metrics.ExponentialTaskLatency, t.timeSource.Since(taskGeneratedStart))
+		t.scope.ExponentialHistogram(metrics.ExponentialTaskProcessingLatency, t.timeSource.Since(taskGeneratedStart))
 	}()
 
 	batchSize := t.dynamicTaskBatchSizer.value()