Promote getAll to TextMapGetter stable API

[jack-berg]

Operation is now stable in the spec: open-telemetry/opentelemetry-specification#4472

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.63%. Comparing base (9cb6365) to head (b2193a6).
Report is 11 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #7267      +/-   ##
============================================
- Coverage     89.92%   89.63%   -0.29%     
- Complexity     6721     6860     +139     
============================================
  Files           765      780      +15     
  Lines         20277    20718     +441     
  Branches       1985     2020      +35     
============================================
+ Hits          18234    18571     +337     
- Misses         1448     1510      +62     
- Partials        595      637      +42     

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