Exports span attributes to ETW

[ThomsonTan]

Changes

Span attributes are current ignored in ETW exporter, this PR add the support.

BTW: if there are multiple writes to the same attribute name, the last one wins.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #1021 (ed834df) into main (55a6517) will not change coverage.
The diff coverage is n/a.

❗ Current head ed834df differs from pull request most recent head 93f15d6. Consider uploading reports for the commit 93f15d6 to get more accurate results

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[lalitb]

LGTM. Thanks for adding. Can we add the test case for this?