Request for Minimum available instances setting

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**Tell us about your request**
What do you want us to build?
Currently there is no minimum available instances setting since the current"MinSize" is just for minimum provisioned instances. This could be a blocker for production services since there is a possibility there there are 0 guaranteed available instances. The customers could potentially get a 5xx error if it scaled to 0. I would like for there to be a "minimum available instances" setting so that there could be a way to prevent it from scaling to 0.

**Describe alternatives you've considered**
The current workaround seem to be just using a script to curl the server every few minutes but this is not ideal and users could get confused with "MinSize"