opentelemetry-api: remove unnecessary copy in iterator (#5288)

* opentelemetry-api: remove unnecessary copy in iterator

The list of keys should be all that's needed for the iterator

Signed-off-by: Alex Boten <223565+codeboten@users.noreply.github.com>

* changelog

Signed-off-by: Alex Boten <223565+codeboten@users.noreply.github.com>

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Signed-off-by: Alex Boten <223565+codeboten@users.noreply.github.com>
Co-authored-by: Leighton Chen <lechen@microsoft.com>