fix: normalize tag labels

[Paul-Bob]

Description

Fixes #2836

Deprecated fetch_labels in order of format_using and ensure the use of correct label when format_using returns a hash with label and value keys.

TODO after approval:

• Document fetch_labels deprecation and suggest format_using usage
• Document possible usages of format_using on tags field
• Tests

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
  • format_using tags docs.avohq.io#223
• I have added tests that prove my fix is effective or that my feature works

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[github-actions]

This PR has been merged into main. The functionality will be available in the next release.

Please check the release guide for more information.