Add e2e test for custom metrics

[skonto]

Changes

• Follows the approach in here.
• Brings in some dependencies from Serving. TODO there is to make some test methods public at the Serving side so code can be removed later on.

Fixes #12

Release Note

Custom metrics are now tested e2e.  

[knative-prow[bot]]

@skonto: The label(s) kind/<kind> cannot be applied, because the repository doesn't have them.

In response to [this](https://github.com/knative-extensions/autoscaler-keda/pull/26): > > > > ># Changes > > > >- TBD >- >- > > >/kind > > >Fixes # > > > >**Release Note** > > >```release-note > >``` > >**Docs** > > >```docs > >``` > Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes-sigs/prow](https://github.com/kubernetes-sigs/prow/issues/new?title=Prow%20issue:) repository.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 63.63%. Comparing base (5c704df) to head (2f0dcc5).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #26 +/- ## =========================================== + Coverage 53.60% 63.63% +10.03% =========================================== Files 5 4 -1 Lines 222 187 -35 =========================================== Hits 119 119 + Misses 95 60 -35 Partials 8 8 ```

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[knative-prow[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ReToCode, skonto

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