Clarify instructions for Managed Prometheus

[zmerlynn]

The instructions linked straight to the installation of the frontend UI, but that skips over the "Before you begin" section of the doc. The "Before you begin" section has a crucial step in it: authorizing the Kubernetes service account to read metrics.

[zmerlynn]

cc @shannonxtreme

[agones-bot]

Build Succeeded 👏

Build Id: e4e6d892-8b4d-4d42-8df9-e448eccd30c2

The following development artifacts have been built, and will exist for the next 30 days:

• image: us-docker.pkg.dev/agones-images/ci/agones-controller:1.31.0-46ea64a-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-sdk:1.31.0-46ea64a-linux-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-ping:1.31.0-46ea64a-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-allocator:1.31.0-46ea64a-amd64
• Linux C++ SDK (build): agonessdk-1.31.0-46ea64a-amd64-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.31.0-46ea64a-amd64.zip

A preview of the website (the last 30 builds are retained):

• https://46ea64a-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/3015/head:pr_3015 && git checkout pr_3015
• helm install agones ./install/helm/agones --namespace agones-system --agones.image.release=us-docker.pkg.dev/agones-images/ci --set agones.image.tag=1.31.0-46ea64a-amd64

[google-oss-prow]

New changes are detected. LGTM label has been removed.

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gongmax, roberthbailey, zmerlynn

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• OWNERS [roberthbailey]

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[agones-bot]

Build Succeeded 👏

Build Id: 1a4a4ec6-2279-47af-975d-0af5b63c77b9

The following development artifacts have been built, and will exist for the next 30 days:

• image: us-docker.pkg.dev/agones-images/ci/agones-controller:1.31.0-2a83508-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-sdk:1.31.0-2a83508-linux-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-ping:1.31.0-2a83508-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-allocator:1.31.0-2a83508-amd64
• Linux C++ SDK (build): agonessdk-1.31.0-2a83508-amd64-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.31.0-2a83508-amd64.zip

A preview of the website (the last 30 builds are retained):

• https://2a83508-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/3015/head:pr_3015 && git checkout pr_3015
• helm install agones ./install/helm/agones --namespace agones-system --agones.image.release=us-docker.pkg.dev/agones-images/ci --set agones.image.tag=1.31.0-2a83508-amd64