docs: add notes on attribute key descriptor

[legendecas]

Currently, in https://github.com/open-telemetry/opentelemetry-js SDK implementation non-own-enumerable keys are ignored. This PR explicitly documents this behavior.

• https://github.com/open-telemetry/opentelemetry-js/blob/main/packages/opentelemetry-sdk-trace-base/src/Span.ts#L115
• https://github.com/open-telemetry/opentelemetry-js/blob/main/packages/opentelemetry-sdk-trace-base/src/Span.ts#L104
• https://github.com/open-telemetry/opentelemetry-js/blob/main/experimental/packages/opentelemetry-sdk-metrics-base/src/utils.ts#L30

[codecov]

Codecov Report

Merging #155 (5a56547) into main (b125324) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #155   +/-   ##
=======================================
  Coverage   95.20%   95.20%           
=======================================
  Files          45       45           
  Lines         646      646           
  Branches      102      102           
=======================================
  Hits          615      615           
  Misses         31       31           

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