[release-1.3] Prober targets service instead of pods directly

[knative-prow-robot]

This is an automated cherry-pick of #2112

Receiver's prober targets services instead of pods directly to allow components to be part of an Istio mesh

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-1.3@1e1ea80). Click here to learn what that means.
The diff coverage is n/a.

@@              Coverage Diff               @@
##             release-1.3    #2159   +/-   ##
==============================================
  Coverage               ?   67.67%           
  Complexity             ?      671           
==============================================
  Files                  ?      141           
  Lines                  ?     6852           
  Branches               ?      196           
==============================================
  Hits                   ?     4637           
  Misses                 ?     1829           
  Partials               ?      386           

Flag	Coverage Δ
java-unittests	82.10% <0.00%> (?)

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[matzew]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: knative-prow-robot, matzew

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[matzew]

/retest-required

[matzew]

@pierDipi what about the release-v1.2 branch?

[aliok]

Manual cherry pick into 1.2 branch: #2180