Support possibility to provide const labels

[FZambia]

A mechanism to enrich metrics by per-node information.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.89%. Comparing base (40d55fc) to head (290007a).
⚠️ Report is 32 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #546      +/-   ##
==========================================
+ Coverage   84.82%   84.89%   +0.07%     
==========================================
  Files          43       43              
  Lines        8452     8486      +34     
==========================================
+ Hits         7169     7204      +35     
+ Misses        872      869       -3     
- Partials      411      413       +2     

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