Rename service to endpoint due to label conflicts

[jonathanio]

Rename the service label to endpoint in order to avoid conflicts with pre-defined Prometheus labels, which is causing service to be renamed to exported_service.

Checklist

Before raising or requesting a review of the pull request, please check and confirm the following items have been performed, where possible:

• I have performed a self-review of my code and run any tests locally to check
• I have added tests that prove that any changes are effective and work correctly
• I have made corresponding changes, as needed, to the repository documentation
• Each commit in, and this pull request, have meaningful subjects and bodies for context
• I have added release/..., type/..., and changes/... labels, as needed, to this pull request

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 9.70%. Comparing base (4b120e2) to head (606d606).
Report is 2 commits behind head on main.

Files	Patch %	Lines
internal/serve/middleware/prometheus.go	0.00%	8 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main     #16   +/-   ##
=====================================
  Coverage   9.70%   9.70%           
=====================================
  Files         17      17           
  Lines        515     515           
=====================================
  Hits          50      50           
  Misses       465     465           

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

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