Add docs for reserved replicas metric

[markmandel]

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:

Noticed there weren't docs for #2609, so here there are!

Which issue(s) this PR fixes:

Work on #2609

Special notes for your reviewer:

N/A

[agones-bot]

Build Failed 😱

Build Id: e45aaadd-8030-43bd-ae4f-1afd4955d651

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[markmandel]

docker run --rm -e "GOOS=darwin" -e "GOARCH=arm64" -e "GO111MODULE=on" -w /go/src/agones.dev/agones -v /workspace/build//.config/gcloud:/root/.config/gcloud -v ~/.kube/:/root/.kube -v ~/.config/helm:/root/.config/helm -v ~/.cache/helm:/root/.cache/helm -v /workspace:/go/src/agones.dev/agones -v /workspace/build//.gomod:/go/pkg/mod -v /workspace/build//.gocache:/root/.cache/go-build agones-build:dea9705952 go build -mod=vendor \
	-o /go/src/agones.dev/agones/cmd/sdk-server/bin/sdk-server.darwin.arm64  -ldflags "-X agones.dev/agones/pkg.Version=1.24.0-0a3f465" agones.dev/agones/cmd/sdk-server
docker run --rm -e "GOOS=darwin" -e "GOARCH=arm64" -e "GO111MODULE=on" -w /go/src/agones.dev/agones -v /workspace/build//.config/gcloud:/root/.config/gcloud -v ~/.kube/:/root/.kube -v ~/.config/helm:/root/.config/helm -v ~/.cache/helm:/root/.cache/helm -v /workspace:/go/src/agones.dev/agones -v /workspace/build//.gomod:/go/pkg/mod -v /workspace/build//.gocache:/root/.cache/go-build agones-build:dea9705952 go build -mod=vendor \
	-o /go/src/agones.dev/agones/cmd/sdk-server/bin/sdk-server.darwin.arm64  -ldflags "-X agones.dev/agones/pkg.Version=1.24.0-0a3f465" agones.dev/agones/cmd/sdk-server

go build agones.dev/agones/cmd/sdk-server: build output "/go/src/agones.dev/agones/cmd/sdk-server/bin/sdk-server.darwin.arm64" already exists and is not an object file

Weird it gets called twice like that.

[agones-bot]

Build Succeeded 👏

Build Id: c8730251-5d2d-44b7-a399-ad070ca733cf

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

• image: gcr.io/agones-images/agones-controller:1.24.0-0a3f465-amd64
• image: gcr.io/agones-images/agones-controller:1.24.0-0a3f465-arm64
• image: gcr.io/agones-images/agones-ping:1.24.0-0a3f465-amd64
• image: gcr.io/agones-images/agones-ping:1.24.0-0a3f465-arm64
• Linux C++ SDK (build): agonessdk-1.24.0-0a3f465-arm64-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.24.0-0a3f465-arm64.zip

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

• https://0a3f465-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/2611/head:pr_2611 && git checkout pr_2611
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.24.0-0a3f465-amd64

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

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

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• OWNERS [markmandel,roberthbailey]

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[govargo]

Thank you!
I'll catch up next.