doc: update metrics scraping method via Prometheus

[niko-lay]

fixes #2140

[CLAassistant]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.56%. Comparing base (65c634f) to head (f8f1c55).
⚠️ Report is 16 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2141   +/-   ##
=======================================
  Coverage   63.56%   63.56%           
=======================================
  Files          32       32           
  Lines        2819     2819           
=======================================
  Hits         1792     1792           
  Misses        901      901           
  Partials      126      126           

Flag	Coverage Δ
integration-test	63.31% <ø> (-0.37%)	⬇️
k8s-integration-test	70.00% <ø> (ø)
oats-test	17.75% <ø> (ø)
unittests	38.52% <ø> (ø)

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[mariomac]

Verified against the actual helm chart and looks correct!