Fix metrics bug for when a gameserver is not retrievable

[pooneh-m]

If the gameserver is not retrievable (e.g. for when a gs is allocated remotely) the operation is logged as warning and fails with nil exception as it continues to gs.Labels.

[agones-bot]

Build Succeeded 👏

Build Id: b9a7b18d-42b9-4eb1-9374-2d78cdf4159e

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

• image: gcr.io/agones-images/agones-controller:1.1.0-e716f0d
• image: gcr.io/agones-images/agones-sdk:1.1.0-e716f0d
• image: gcr.io/agones-images/agones-ping:1.1.0-e716f0d
• Linux C++ SDK (build): agonessdk-1.1.0-e716f0d-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.1.0-e716f0d.zip

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

• https://e716f0d-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/GoogleCloudPlatform/agones.git pull/1101/head:pr_1101 && git checkout pr_1101
• helm install install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.1.0-e716f0d

[google-oss-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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