Add Unit Tests that verify stats reporting in autoscaler.

[vagababov]

In what area(s)?

/area autoscale
/area test-and-release

Other classifications:
/kind good-first-issue

Current unit tests for autoscaler do not verify that the autoscaler actually reported the metrics we expect it to.
A test (or set of tests) to verify that this works as expected is needed.

[MIBc]

I can have a try.

[vagababov]

/assign @MIBc

[MIBc]

What kind of unit test do we need to verify metrics autoscaler reported. Such as stable_request_concurrency in knative.dev/serving/pkg/autoscaler/scaling/metrics.go ? @vagababov

[vagababov]

you call scale for various initial settings and verify that #pods, panic and stable concurrency and rps and excess burst capacity are all correctly reported.

[vagababov]

@MIBc how is this progressing? This has given us trouble already and I'd like to be done before the rease cuts.

[MIBc]

Sorry for delay fixing this issue. I have no idea how to get the metrics autoscaler report.