fix: TraceState cannot insert new key-value pairs.

[TommyCpp]

fix #566

[codecov]

Codecov Report

Merging #567 (a8210e6) into main (dc7d81f) will increase coverage by 0.0%.
The diff coverage is 100.0%.

❗ Current head a8210e6 differs from pull request most recent head 71762f3. Consider uploading reports for the commit 71762f3 to get more accurate results

@@          Coverage Diff          @@
##            main    #567   +/-   ##
=====================================
  Coverage   54.7%   54.7%           
=====================================
  Files         98      98           
  Lines       8576    8586   +10     
=====================================
+ Hits        4694    4704   +10     
  Misses      3882    3882           

Impacted Files	Coverage Δ
opentelemetry/src/sdk/trace/tracer.rs	85.9% <100.0%> (ø)
opentelemetry/src/trace/span_context.rs	92.4% <100.0%> (+0.6%)	⬆️
opentelemetry/src/context.rs	92.1% <0.0%> (-0.8%)	⬇️

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[TommyCpp]

@jtescher While I am working on it. I also added a new Error type for TraceStateError. Will try to eliminate other places where the returned error type are () in the future PRs.

[jtescher]

LGTM