Checklist:

• I've included steps to reproduce the bug.
• I've included the version of argo rollouts.

Describe the bug

In #119, a defaulting of missing replicas: field to 1 was added. This behaviour could impact reliability of a service. Defaulting was added to workaround HPA's deficiency (kubernetes/kubernetes#111781)

We are using a custom autoscaler, not HPA. Our deployments tend not to have replicas: field defined. Our expectations is that during new version rollout, the number of replicas is carried-over from previous version.

What we observed, when replicas: field is not set, argo-rollouts sets it to 1. After few seconds, our autoscaler sets replicas: back to the proper value. Unfortunately during those few seconds Kubernetes starts killing the pods, and until new pods are spun up and ready, the service is degraded.

To Reproduce

Prepare a deployment without a spec.replicas: field. Observe "Defaulting .spec.replica to 1" message in argo rollout's log.

Expected behavior

The number of replicas, if not defined, should be carried over from current state. The code should do something like

	// In order to work with HPA, the rollout.Spec.Replica field cannot be nil. As a result, the controller will update
	// the rollout to have the replicas field set to the current or the default value. see https://github.com/argoproj/argo-rollouts/issues/119
	if rollout.Spec.Replicas == nil {
		if rollout.Status.Replicas != nil {
			replicas := rollout.Status.Replicas
		} else {
			replicas := pointer.Int32Ptr(defaults.DefaultReplicas)
		}
		logCtx.Info("Defaulting .spec.replicas to %d", replicas)
		r.Spec.Replicas = replicas

(Above isn't syntactically correct, sorry).

rollout.status.replicas: is NOT marked as an optional field, so should be available all the time.
The value of 0 (zero) of the number of status replicas may be wanted/correct and should be carried over, too.

Screenshots

Version
1.7.0

Logs

# Paste the logs from the rollout controller

# Logs for the entire controller:
kubectl logs -n argo-rollouts deployment/argo-rollouts

# Logs for a specific rollout:
kubectl logs -n argo-rollouts deployment/argo-rollouts | grep rollout=<ROLLOUTNAME

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