Tracestate: Keep last value for duplicate key

[tsloughter]

This was recently updated in the W3C spec and they added tests we had to disable in order to pass because of the change. This patch fixes the decoding to keep the last value for any duplicate keys encountered and enables the strictest level of w3c interop tests.

Also a few patches that cleanup some docs stuff.

[codecov]

Codecov Report

Merging #346 (4cc68b0) into main (3e3281c) will increase coverage by 0.21%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #346      +/-   ##
==========================================
+ Coverage   33.33%   33.55%   +0.21%     
==========================================
  Files          56       56              
  Lines        4563     4557       -6     
==========================================
+ Hits         1521     1529       +8     
+ Misses       3042     3028      -14     

Flag	Coverage Δ
api	68.90% <100.00%> (+1.62%)	⬆️
elixir	16.47% <0.00%> (+0.02%)	⬆️
erlang	33.49% <100.00%> (+0.22%)	⬆️
exporter	21.47% <100.00%> (+<0.01%)	⬆️
sdk	77.13% <ø> (ø)
zipkin	2.59% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
apps/opentelemetry_api/lib/open_telemetry.ex	21.42% <ø> (ø)
...elemetry_api/src/otel_propagator_trace_context.erl	70.83% <100.00%> (+19.81%)	⬆️
...ntelemetry_exporter/src/opentelemetry_exporter.erl	75.77% <100.00%> (+1.68%)	⬆️

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