Add metric for number of reserved replicas

[govargo]

What type of PR is this?

/kind feature

What this PR does / Why we need it:

The current metrics for fleets include total game servers, allocated game servers, ready game servers, and desired game servers, however, doesn't include reserved game servers.
This PR adds the count metrics for reserved game servers.

Which issue(s) this PR fixes:

Closes #2609

Special notes for your reviewer:
None

[agones-bot]

Build Succeeded 👏

Build Id: fe3a4987-36f6-4963-a010-c8be12e2ca73

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-d422929-amd64
• image: gcr.io/agones-images/agones-controller:1.24.0-d422929-arm64
• image: gcr.io/agones-images/agones-ping:1.24.0-d422929-amd64
• image: gcr.io/agones-images/agones-ping:1.24.0-d422929-arm64
• Linux C++ SDK (build): agonessdk-1.24.0-d422929-arm64-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.24.0-d422929-arm64.zip

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

• https://d422929-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/2610/head:pr_2610 && git checkout pr_2610
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.24.0-d422929-amd64

[govargo]

I added to unit test and tried on GKE cluster with reserved game server.

Here is the screenshot of Prometheus graph(type=reserved).

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

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

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• OWNERS [roberthbailey]

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[rcreasey]

Looks great! Thanks for knocking this out!