Cleanup UID/InstanceID duplicity. Also remove override_instance_id config option

[mariomac]

Having UID and Instance fields in the svc.ID allowed users to override the instance ID. This was created as a troubleshooting feature that would only work when the Beyla instance instrumented a single application instance (sidecar mode).

This feature adds some complexity to the code, but still has remained unused (both because we haven't required to troubleshoot instance IDs, and because Beyla usually instruments multiple instances, so using this feature would break the instrumentation).

Removing the duplicity, and the overriding feature, fixes one of the two described duplicities in the issue #1124

After this is merged, the simpler code would facilitate removing the other duplicity from the aforementioned issue.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.05%. Comparing base (9a20dda) to head (db24bed).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/internal/traces/read_decorator.go	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1125      +/-   ##
==========================================
+ Coverage   82.03%   82.05%   +0.01%     
==========================================
  Files         140      140              
  Lines       11574    11562      -12     
==========================================
- Hits         9495     9487       -8     
+ Misses       1557     1553       -4     
  Partials      522      522              

Flag	Coverage Δ
integration-test	57.46% <90.90%> (+0.14%)	⬆️
k8s-integration-test	59.26% <90.90%> (-0.01%)	⬇️
oats-test	36.87% <63.63%> (-0.03%)	⬇️
unittests	52.27% <72.72%> (-0.09%)	⬇️

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[marctc]

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