Support scale to zero

[skonto]

Changes

• Allows to scale to zero based on an external metrics. KEDA de-activates hpa by setting its targets to unknown.

$ k get hpa -n serving-tests
NAME                           REFERENCE                                            TARGETS       MINPODS   MAXPODS   REPLICAS   AGE
scale-to-zero-gotnxmxb-00001   Deployment/scale-to-zero-gotnxmxb-00001-deployment   <unknown>/5   1         10        0          6m53s

Similarly the scaledobject shows to be inactive.

$k get scaledobject -n serving-tests
NAME                           SCALETARGETKIND      SCALETARGETNAME                           MIN   MAX   TRIGGERS     AUTHENTICATION   READY   ACTIVE   FALLBACK   PAUSED    AGE
scale-to-zero-lllzzbds-00001   apps/v1.Deployment   scale-to-zero-lllzzbds-00001-deployment         10    prometheus                    True    False    False      Unknown   6m14s

• Tested locally:

stream.go:304: I 12:57:22.399 controller-6f874b497b-jcwkt [knative.dev.serving.pkg.reconciler.gc.reconciler] [serving-tests/scale-to-zero-gotnxmxb] Reconcile succeeded
stream.go:304: I 12:57:22.399 controller-6f874b497b-jcwkt [knative.dev.serving.pkg.reconciler.configuration.Reconciler] [serving-tests/scale-to-zero-gotnxmxb] Reconcile succeeded
--- PASS: TestScaleToZero (430.80s)
PASS
ok  	knative.dev/autoscaler-keda/test/e2e	430.821s

/kind enhancement

Fixes #15

Release Note

Support scale to zero. When we scale down to zero external metrics need to be available for scaling up again. Initially when a service is created it will not become ready unless hpa reports its target correctly (external metrics are available).  

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.77%. Comparing base (c2fa22d) to head (13881fe).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #55      +/-   ##
==========================================
- Coverage   73.46%   72.77%   -0.69%     
==========================================
  Files           8        8              
  Lines         520      518       -2     
==========================================
- Hits          382      377       -5     
- Misses        108      110       +2     
- Partials       30       31       +1     

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[skonto]

We can't get the resources from service status as it is not ready until we get metrics. So we just wait. Can be improved in another PR to actually fetch the objects when they are ready.

[ReToCode]

/lgtm
/approve

[knative-prow]

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