Service Initial Scale

[taragu]

/area API
/area autoscale

Proposal: https://docs.google.com/document/d/1VG1aR9bHsQZbU3hRkMPnKyCcj7loCl5imBu3_EQ3Jdw/edit

Describe the feature

This issue is evolved from #4098. We'd like to allow operators and users to indicate the initial scale of a new service. We are introducing two operator knobs in config-autoscaler:

• default-initial-scale identifies the default scale for all new Revisions upon creation.
• allow-zero-initial-scale indicates whether value 0 is allowed as in the Revision annotation.

We also introduce an annotation on Revisions called autoscaling.internal.knative.dev/initialScale, which specifies the initial scale of the Revision. Its implied value is default-initial-scale in config-autoscaler.

TODO:

• Implementation
• e2e tests
• Documentation

[vagababov]

/assign taragu