update docs

[rileyhun]

What this PR does / why we need it:

Provides documentation that addresses issue that occurred when set up Istio AuthorizationPolicy that subsequently blocked Prometheus Server from scraping metrics

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[seldondev]

Hi @rileyhun. Thanks for your PR.

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[seldondev]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[ukclivecox]

@rileyhun Thanks!