Update path label to better handle 404 requests

[jonathanio]

Update the processing of the path label to move an empty path to a 404 value, to better represent that the path was not found by a handler, but processed by the NotFound handler instead.

Checklist

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• I have performed a self-review of my code and run any tests locally to check
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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 9.70%. Comparing base (a6ff104) to head (277dfb7).
Report is 2 commits behind head on main.

Files	Patch %	Lines
internal/serve/middleware/prometheus.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main     #15      +/-   ##
========================================
- Coverage   9.74%   9.70%   -0.04%     
========================================
  Files         17      17              
  Lines        513     515       +2     
========================================
  Hits          50      50              
- Misses       463     465       +2     

Flag	Coverage Δ
unit-tests	9.70% <0.00%> (-0.04%)	⬇️

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Files	Coverage Δ
internal/serve/middleware/prometheus.go	0.00% <0.00%> (ø)