Resource.get modified to only need a reference

[cijothomas]

Minor change, also few nit doc updates.
Such get() methods typically only requires ref, so modified Resource::get to also match that.

[codecov]

Codecov Report

Attention: Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.

Project coverage is 77.8%. Comparing base (4f472a8) to head (15d18a7).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
opentelemetry-zipkin/src/exporter/mod.rs	0.0%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2552   +/-   ##
=====================================
  Coverage   77.8%   77.8%           
=====================================
  Files        122     122           
  Lines      23055   23058    +3     
=====================================
+ Hits       17952   17955    +3     
  Misses      5103    5103           

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