Add comment why histogram might already exists in metricsprocessor

[krdln]

cc @DariaKunoichi @kwapik

---

This change is 

[codecov]

Codecov Report

Base: 49.28% // Head: 49.24% // Decreases project coverage by -0.04% ⚠️

Coverage data is based on head (0a52f82) compared to base (7ac6103).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #250      +/-   ##
==========================================
- Coverage   49.28%   49.24%   -0.05%     
==========================================
  Files         226      226              
  Lines       18850    18854       +4     
==========================================
- Hits         9291     9285       -6     
- Misses       8963     8971       +8     
- Partials      596      598       +2     

Impacted Files	Coverage Δ
pkg/otelcollector/metricsprocessor/processor.go	71.02% <0.00%> (-1.36%)	⬇️
...ne/actuators/concurrency/scheduler/token-bucket.go	95.60% <0.00%> (-4.40%)	⬇️
...s/dataplane/actuators/concurrency/scheduler/wfq.go	98.65% <0.00%> (-1.35%)	⬇️
operator/controllers/agent_reconciler.go	48.30% <0.00%> (+0.48%)	⬆️

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