Fix default resource detection for tracer provider

[srikanthccv]

Spec says

The SDK MUST extract information from the OTEL_RESOURCE_ATTRIBUTES environment variable and merge this, as the secondary resource, with any resource information provided by the user, i.e. the user provided resource information has higher priority.

[codecov]

Codecov Report

Merging #641 (7c271e5) into main (542c28a) will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #641      +/-   ##
==========================================
+ Coverage   61.98%   62.08%   +0.10%     
==========================================
  Files          95       95              
  Lines        7683     7705      +22     
==========================================
+ Hits         4762     4784      +22     
  Misses       2921     2921              

Impacted Files	Coverage Δ
opentelemetry/src/sdk/resource/mod.rs	87.38% <ø> (ø)
opentelemetry/src/sdk/trace/provider.rs	84.82% <100.00%> (+2.86%)	⬆️
opentelemetry/src/sdk/trace/span_processor.rs	82.18% <0.00%> (-0.07%)	⬇️

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[TommyCpp]

Looks good to me. Thanks! really love the unit tests.

[TommyCpp]

@jtescher Any more comments or it's good to merge?

[jtescher]

@TommyCpp one last issue to resolve one way or the other