Added back metrics support for extensions

[chiayi]

Add back metrics support within extensions

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 back the metrics support within Extensions that was removed during the trimming process.

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

Special notes for your reviewer:

[agones-bot]

Build Failed 😱

Build Id: 247050d4-fe47-4e38-8914-225864be2901

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[agones-bot]

Build Succeeded 👏

Build Id: d1a3d142-14b6-4303-99bd-04e3b7200f8e

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:1.30.0-69c3c69-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-sdk:1.30.0-69c3c69-linux-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-ping:1.30.0-69c3c69-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-allocator:1.30.0-69c3c69-amd64
• Linux C++ SDK (build): agonessdk-1.30.0-69c3c69-amd64-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.30.0-69c3c69-amd64.zip

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

• https://69c3c69-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/2988/head:pr_2988 && git checkout pr_2988
• helm install agones ./install/helm/agones --namespace agones-system --agones.image.release=us-docker.pkg.dev/agones-images/ci --set agones.image.tag=1.30.0-69c3c69-amd64

[google-oss-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: chiayi, zmerlynn
Once this PR has been reviewed and has the lgtm label, please assign cyriltovena for approval. For more information see the Kubernetes Code Review Process.

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[agones-bot]

Build Succeeded 👏

Build Id: 5a21ccbf-b306-4059-b47a-5f3ea2878509

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-f14dae4-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-sdk:1.30.0-f14dae4-linux-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-ping:1.30.0-f14dae4-amd64
• image: us-docker.pkg.dev/agones-images/ci/agones-allocator:1.30.0-f14dae4-amd64
• Linux C++ SDK (build): agonessdk-1.30.0-f14dae4-amd64-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.30.0-f14dae4-amd64.zip

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

• https://f14dae4-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/2988/head:pr_2988 && git checkout pr_2988
• 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-f14dae4-amd64