factored metrics logic out of the allocator main file

[pooneh-m]

What type of PR is this?
/kind cleanup

What this PR does / Why we need it:
Moved metrics related code out of agones-allocator main.go to its owns separate file.

[agones-bot]

Build Succeeded 👏

Build Id: 3806eddf-57a8-4219-8c8f-951732ec6fea

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

• image: gcr.io/agones-images/agones-controller:1.6.0-2cb6393
• image: gcr.io/agones-images/agones-sdk:1.6.0-2cb6393
• image: gcr.io/agones-images/agones-ping:1.6.0-2cb6393
• Linux C++ SDK (build): agonessdk-1.6.0-2cb6393-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.6.0-2cb6393.zip

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

• https://2cb6393-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/GoogleCloudPlatform/agones.git pull/1512/head:pr_1512 && git checkout pr_1512
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.6.0-2cb6393

[cyriltovena]

LGTM

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cyriltovena, pooneh-m

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