Allow KnativeService update to stay to same scale, eg: Initial-scale=previous

[pastjean]

/area autoscale

Describe the feature

It would be nice to be able to update service without changing their current scale

Situation:

So we have a bunch of knative services ( > 1800 ). With all of them having scale to zero enabled. They all do have different different docker image but they do share code. It's common to update that shared code sometimes and update all those knative service.

• allow-scale-to-zero: "true"
• Service 1 initial-scale = 1, current-scale = 3
• Service 2 initial-scale = 1, current-scale = 0

When updating the Services they all go back to their initial-scale instead of staying at their current scale. Meaning:

• Service 1 = current-scale = 1
• Service 2 = current-scale = 1

Even though autoscaler knows it needs more of Service 1  and none of Service 2.

My feature request: have a way to tell the autoscaler to put the initialScale to something like previous or current

See slack thread about the history on this: https://knative.slack.com/archives/C0186KU7STW/p1662038186291709

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[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Reopen the issue with /reopen. Mark the issue as
fresh by adding the comment /remove-lifecycle stale.