fix: Update metrics catalog

[kafeelhasan]

Proposed changes

Change Type to average for system.cpu Metric for nginx azure in metrics-catalog.

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[github-actions]

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[russokj]

Looks good. Just a minor suggestion.

[arpith-f5]

LGTM! Let's wait for LIFTR to deploy the new metric specification before we update the docs.

[ADubhlaoich]

LGTM!