Add metric for instrumented processes

[marctc]

This PR adds a Gauge to track all services registered by Beyla.
Useful for debugging purpouses. Displays something like this:

beyla_instrumented_processes{service="php"} 1
beyla_instrumented_processes{service="productcatalogservice"} 1
beyla_instrumented_processes{service="prometheus"} 1
beyla_instrumented_processes{service="python3.12"} 2

Current limtation is for intepreted languanges such Python, Ruby or Java, as we won't visualize
which program are we discovering.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 79.31%. Comparing base (6c04f76) to head (a166014).
Report is 4 commits behind head on main.

Files	Patch %	Lines
pkg/internal/imetrics/iprom.go	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #928      +/-   ##
==========================================
+ Coverage   79.16%   79.31%   +0.15%     
==========================================
  Files         131      131              
  Lines        9948    10139     +191     
==========================================
+ Hits         7875     8042     +167     
- Misses       1569     1586      +17     
- Partials      504      511       +7     

Flag	Coverage Δ
integration-test	55.27% <73.33%> (+0.40%)	⬆️
k8s-integration-test	59.20% <86.66%> (-0.08%)	⬇️
oats-test	35.50% <13.33%> (-0.95%)	⬇️
unittests	47.19% <0.00%> (+1.73%)	⬆️

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[grcevski]

LGTM! What do you think about extending one of our integration tests to ensure this works? Maybe one of those which test multiple processes, then you can enable the internal metrics and confirm we see all services we expect to see?