Fix missing service info

[grcevski]

This PR fixes two bugs in the k8s service discovery:

1. We had an early return in the loop that lists services, instead of a continue on a headless service.
2. While eyeballing the code when looking for the bug, we noticed a slight timing hole with the cache, which was unlikely the culprit but it's good to fix.

Closes #933
Closes #1426

I'll backport this to 1.9

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.94%. Comparing base (a6d91fd) to head (c3a939e).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/kubecache/meta/informers.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1444      +/-   ##
==========================================
+ Coverage   76.90%   80.94%   +4.04%     
==========================================
  Files         149      149              
  Lines       15250    15252       +2     
==========================================
+ Hits        11728    12346     +618     
+ Misses       2906     2298     -608     
+ Partials      616      608       -8     

Flag	Coverage Δ
integration-test	59.78% <0.00%> (-0.08%)	⬇️
k8s-integration-test	60.32% <80.00%> (?)
oats-test	33.91% <0.00%> (-0.01%)	⬇️
unittests	51.94% <80.00%> (-0.02%)	⬇️

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[rafaelroquetto]

Good stuff!

[mariomac]

Good catch!

[mariomac]

NIT: This operation is read-only. Why do you wrap into the mutex Write lock?

[grcevski]

I think you are right, we don't need that one under the lock, the read after was important to be inside the write lock.