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]

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

In response to this:

Changes

• TBD

/kind

Fixes #

Release Note

Docs

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@             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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[ReToCode]

Looks good 👍

/lgtm
/approve

[knative-prow]

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