Update Knative Prow to use the new monitoring stack

[chizhg]

As per the Prow owners, Knative Prow is currently using a legacy monitoring stack (in https://github.com/knative/test-infra/tree/5094d5004d5b9188e508024789d0e0dee2b1a866/prow/cluster/monitoring), which has been deprecated in favor of https://github.com/GoogleCloudPlatform/oss-test-infra/tree/master/prow/oss/terraform.

Since the legcy monitoring stack is not actively being maintained, and the new one is easier to setup and measure more valid metrics, Prow team's recommendation is to make the switch to it.

Please reach out to Prow team via the k8s/Prow slack channel if there are any questions.

/cc @kvmware @upodroid

[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Reopen the issue with /reopen. Mark the issue as
fresh by adding the comment /remove-lifecycle stale.

[cardil]

/remove-lifecycle stale
/reopen
/triage accepted

[knative-prow]

@cardil: Reopened this issue.

In response to this:

/remove-lifecycle stale
/reopen
/triage accepted

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