add get_description to sampler and support for updating tracestate in sampler

[tsloughter]

No description provided.

[codecov]

Codecov Report

Merging #129 into master will increase coverage by 0.20%.
The diff coverage is 88.00%.

@@            Coverage Diff             @@
##           master     #129      +/-   ##
==========================================
+ Coverage   31.83%   32.03%   +0.20%     
==========================================
  Files          63       63              
  Lines        2972     2981       +9     
==========================================
+ Hits          946      955       +9     
  Misses       2026     2026              

Flag	Coverage Δ
#api	46.44% <ø> (ø)
#elixir	16.24% <ø> (ø)
#erlang	31.87% <88.00%> (+0.21%)	⬆️
#exporter	17.05% <ø> (ø)
#sdk	64.37% <88.00%> (+0.46%)	⬆️

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Impacted Files	Coverage Δ
apps/opentelemetry/src/otel_span_utils.erl	87.50% <60.00%> (ø)
apps/opentelemetry/src/otel_sampler.erl	84.61% <95.00%> (+1.28%)	⬆️
apps/opentelemetry/src/otel_batch_processor.erl	73.56% <0.00%> (+1.14%)	⬆️

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