changed error to warning

[KalmanMeth]

If we use multiple encode_prom stages, each instance tries to register its operational metrics. This results in a prometheus.AlreadyRegisteredError, which is not really an error, so we convert it to a warning.

[openshift-ci]

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[codecov]

Codecov Report

Merging #443 (248f28f) into main (f8a40f8) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #443      +/-   ##
==========================================
- Coverage   64.15%   64.12%   -0.03%     
==========================================
  Files          96       96              
  Lines        6723     6723              
==========================================
- Hits         4313     4311       -2     
- Misses       2165     2166       +1     
- Partials      245      246       +1     

Flag	Coverage Δ
unittests	64.12% <100.00%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/operational/metrics.go	75.65% <100.00%> (ø)

... and 1 file with indirect coverage changes

[jpinsonneau]

LGTM thanks @KalmanMeth !