Update tracing docs and add note to the @distributed_trace decorator that it's not intended for end users

[lmolkova]

Fixes #37342

Updates tracing plugin docs to use AzMon OTel distro instead of exported.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

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• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[azure-sdk]

API change check

API changes are not detected in this pull request.

[pvaneck]

Thanks for this! Seems our "Verify Readmes" check isn't happy with the lack of "Key concepts", "Examples", and "Next steps" sections in the README file. I think we can either squeeze the section headings in, or perhaps just add the README path here. Other core libraries don't necessarily conform to these headings anyway.

[pvaneck]

Identity CI failures were addressed in main and can be overridden.

[lmolkova]

thank you @pvaneck !