Update metrics-server guide to include aws-auth step

[bagf]

Description of changes:

Include missing step in the metric-server guide, required to authenticate users and/or roles.

Without this additional step, running the following command results in a unauthorized error:

kubectl top nodes
error: You must be logged in to the server (Unauthorized)

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[joshbean]

Closing this issue or pull request in advance of archiving this repo. For more information about the decision to archive this repo (and others in the 'awsdocs' org), see the announcement on the AWS News Blog.

[geoffcline]

Thank you for your feedback @bagf - I have staged changes to address this issue.