NGINXaaS Platform metric migration

[arpith-f5]

Proposed changes

Write a clear and concise description that helps reviewers understand the purpose and impact of your changes. Use the
following format:

Problem: Add instructions for customer to migrate from custom metric to platform metrics for NGINXaaS

Solution: created a new page with instructions.

Testing: Local testing.

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specific feedback, add them here.

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Closes #ISSUE

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[arpith-f5]

I have hereby read the F5 CLA and agree to its terms

[amudukutore]

@arpith-f5 - Thanks for making these changes. These instructions will be very useful as we guide customers to switch over to the new metrics mechanism.