[WIP] Define autoscaling for Knative Eventing

[aslom]

Problem
Users of Knative Eventing expect that autoscaling is available and works. We should define what autoscaling for Knative Eventing means and how user experience would work to make it serverless and Knative-way.

Persona:
All personas

Exit Criteria
Documented what autoscaling should be in Knative Eventing

Time Estimate (optional):
2w

Additional context (optional)
This is outcome of discussion about building autoscaling annotations where we realized that before we define technical mechanisms (such as using annotations) we need first to define what we mean by "autoscaling" in Knative Eventing

[aslom]

Please view, comment and edit proposals for autoscaling goals in public google doc:
https://docs.google.com/document/d/1usNmsuHBWzVaL5GGC873iGVrkKXGbc6t7bLHJL38Cyg/edit?usp=sharing

/cc @grantr @vaikas @matzew @lionelvillard

[grantr]

@aslom is there a single tracking issue that we can link all the autoscaling bugs to? That will make these easier to triage. Thanks!

[lberk]

Hi, what is the overlap (and can we get a single tracker for) all these related issues?
#2161
#2154
#2153
#2152

[aslom]

@lberk they all talk about "scaling" different part of KnE - I hope we can reach here consensus on definition of (auto)scaling in Knative Eventing and then apply it to plan what we do those parts

[grantr]

Sounds like you'd like this issue to be the overall tracking issue then?

[aslom]

Putting together google doc to summarize options: https://docs.google.com/document/d/18Ma-uqsB0SYzwvRZ_rz5c8m21JAXGP0oLZpr8W0Jwak/edit?usp=sharing

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