fix: Report error using OTel convention

[cijothomas]

Fixes #2150
Only does "exception.message", as I don't see any way to report other attributes. This may be improved in the future.
Also updated to map u128 and i128.
This is the last change planned before stable release of the crate.

[codecov]

Codecov Report

Attention: Patch coverage is 97.05882% with 2 lines in your changes missing coverage. Please review.

Project coverage is 80.5%. Comparing base (bece03b) to head (fc310aa).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-appender-tracing/src/layer.rs	97.0%	2 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2808   +/-   ##
=====================================
  Coverage   80.5%   80.5%           
=====================================
  Files        124     124           
  Lines      23411   23477   +66     
=====================================
+ Hits       18849   18913   +64     
- Misses      4562    4564    +2     

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