etcdctl: fix json output

[kstrifonoff]

Fix #19262

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @kstrifonoff. Thanks for your PR.

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[kstrifonoff]

/cc @ivanvc

[ivanvc]

Hi @kstrifonoff, thanks for the pull request. I'm respectful to the original contributor who asked to work on this. Our guideline is to re-assign after a week of no answer when asking a contributor for updates.

A couple of suggestions by quickly looking at the code.

1. The imports contain a lint error. Running make verify-lint locally will show you the error.
2. The code doesn't have unit tests for this, but there's an e2e test, TestCtlV3MemberListWithHex. I think the test should pass, but adding your new cases would be good.
3. I suggest reviewing the code from printKV. I think it would be better to follow that implementation approach, i.e., there would be less repetition with the if conditions.

Thanks again for your pull request :)