Add Extensions Resource Dashboard

[chiayi]

What type of PR is this?

Uncomment only one /kind <> line, press enter to put that in a new line, and remove leading whitespace from that line:

/kind breaking
/kind bug
/kind cleanup
/kind documentation

/kind feature

/kind hotfix

What this PR does / Why we need it:
This adds a Grafana dashboard for Extensions resource usage.

Which issue(s) this PR fixes:
Work on #2797

Special notes for your reviewer:
Screenshot of the dashboard:

[markmandel]

Are there new metrics we should document here: https://agones.dev/site/docs/guides/metrics/ ?

[agones-bot]

Build Succeeded 👏

Build Id: 6eaf8273-ae7a-4cd6-a848-229312053c8d

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:1.30.0-75ebe45-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-sdk:1.30.0-75ebe45-linux-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-ping:1.30.0-75ebe45-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-allocator:1.30.0-75ebe45-amd64
• Linux C++ SDK (build): agonessdk-1.30.0-75ebe45-amd64-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.30.0-75ebe45-amd64.zip

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

• https://75ebe45-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/2993/head:pr_2993 && git checkout pr_2993
• helm install agones ./install/helm/agones --namespace agones-system --agones.image.release=us-docker.pkg.dev/agones-images/ci --set agones.image.tag=1.30.0-75ebe45-amd64

[chiayi]

Still need to add to metrics docs. But there are not any new metrics that need to be documented, just different parameters.

[zmerlynn]

Are there new metrics we should document here: https://agones.dev/site/docs/guides/metrics/ ?

There will be new pods with new labels (e.g. agones_dev_role of extensions) but the metrics are the same. They're just forked, streams, basically.

[markmandel]

Got it - SGTM!

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chiayi, markmandel

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[zmerlynn]

Taking a sec to test that this works on my setup, too, for curiousity.