Add ReadableSpan#getAttributes

[jack-berg]

Resolves #6344.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 91.10%. Comparing base (a85e888) to head (5d589c4).
Report is 3 commits behind head on main.

Files	Patch %	Lines
.../java/io/opentelemetry/sdk/trace/ReadableSpan.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6382      +/-   ##
============================================
+ Coverage     91.08%   91.10%   +0.02%     
- Complexity     5771     5774       +3     
============================================
  Files           627      627              
  Lines         16852    16855       +3     
  Branches       1720     1721       +1     
============================================
+ Hits          15349    15356       +7     
+ Misses         1008     1006       -2     
+ Partials        495      493       -2     

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