Use Sets for traces_target_info metric

[marctc]

Previous implementation was using Add and Sub for traces_target_info.
This is risky because in some cases, it can lead to negative values.

Fixes #1707

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 67.01%. Comparing base (52143b0) to head (d06e090).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/export/prom/prom.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1735      +/-   ##
==========================================
+ Coverage   66.95%   67.01%   +0.05%     
==========================================
  Files         214      214              
  Lines       22225    22225              
==========================================
+ Hits        14880    14893      +13     
+ Misses       6607     6598       -9     
+ Partials      738      734       -4     

Flag	Coverage Δ
integration-test	54.21% <50.00%> (+0.09%)	⬆️
k8s-integration-test	53.86% <50.00%> (-0.02%)	⬇️
oats-test	34.86% <0.00%> (+0.11%)	⬆️
unittests	43.87% <0.00%> (+<0.01%)	⬆️

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