Test cleanups

[lukasmalkmus]

This PR performs mostly code grooming. It resolves some outstanding TODOs, improves GoDoc, makes testing for adapters simpler and does some smaller cleanups in test files.

Needs #141.

[codecov]

Codecov Report

Base: 72.20% // Head: 71.22% // Decreases project coverage by -0.98% ⚠️

Coverage data is based on head (825bd65) compared to base (643f6e2).
Patch coverage: 70.92% of modified lines in pull request are covered.

❗ Current head 825bd65 differs from pull request most recent head 845cb93. Consider uploading reports for the commit 845cb93 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #142      +/-   ##
==========================================
- Coverage   72.20%   71.22%   -0.99%     
==========================================
  Files          36       42       +6     
  Lines        1684     1835     +151     
==========================================
+ Hits         1216     1307      +91     
- Misses        353      399      +46     
- Partials      115      129      +14     

Impacted Files	Coverage Δ
adapters/apex/apex.go	77.06% <ø> (-0.42%)	⬇️
adapters/logrus/logrus.go	74.57% <ø> (-0.43%)	⬇️
axiom/error.go	64.70% <ø> (ø)
axiom/axiom.go	50.00% <40.00%> (-30.00%)	⬇️
internal/config/defaults.go	42.85% <42.85%> (ø)
axiom/otel/trace_config.go	53.84% <53.84%> (ø)
axiom/otel/trace.go	70.37% <70.37%> (ø)
axiom/client.go	81.25% <72.22%> (-3.48%)	⬇️
internal/config/option.go	72.72% <72.72%> (ø)
internal/config/config.go	77.77% <77.77%> (ø)
... and 4 more

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[bahlo]

💯

[github-actions]

This PR/issue depends on:

• Helper package for OpenTelemetry Traces #141
  By Dependent Issues (🤖). Happy coding!