Apply Splunk span limit defaults

[MrAlias]

Conform with the GDI specification:

• Distribution MUST default to 1000 for OTEL_SPAN_LINK_COUNT_LIMIT (not OpenTelemetry default)
• Distribution MUST default to 12000 for OTEL_ATTRIBUTE_VALUE_LENGTH_LIMIT (not OpenTelemetry default)
• Distribution MUST be unset "" (unlimited) for all others (not OpenTelemetry default)

[codecov-commenter]

Codecov Report

Merging #723 (f3f50bb) into main (8ff26ac) will increase coverage by 0.47%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #723      +/-   ##
==========================================
+ Coverage   78.87%   79.34%   +0.47%     
==========================================
  Files          55       56       +1     
  Lines        2755     2818      +63     
==========================================
+ Hits         2173     2236      +63     
  Misses        533      533              
  Partials       49       49              

Flag	Coverage Δ
Linux	79.34% <100.00%> (+0.47%)	⬆️
macOS	75.65% <100.00%> (?)

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Impacted Files	Coverage Δ
distro/config.go	100.00% <100.00%> (ø)
distro/limits.go	100.00% <100.00%> (ø)
distro/otel.go	64.40% <100.00%> (+0.61%)	⬆️

---

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[pellared]

It would be good to mention the env vars in the markdown docs. But it can be done later. Approving and merging.