Default resource detector env var and add auto-instrumentation resource detector

[jeremydvoss]

Description

Follow up to #33305 and fix for #33295, and #33253. Changed resource detector behavior to only default the env var. This will allow customers to customize resource detectors and disable any that may cause issues.

Also, adding the app service detector to auto-insturmentation

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• The pull request does not introduce [breaking changes]
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[azure-sdk]

API change check

API changes are not detected in this pull request.