Added several tests for metrics package

[akremsa]

What type of PR is this?

/kind cleanup

What this PR does / Why we need it:
Added several tests for metrics package, improved log messages.

[agones-bot]

Build Failed 😱

Build Id: 38f9b2b4-2526-4e61-b533-7e5a9ddb87cf

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• Cloud Build log download

To get permission to view the Cloud Build view, join the agones-discuss Google Group.

[agones-bot]

Build Succeeded 👏

Build Id: dd6475c1-e513-4e76-bc76-06ade81167e3

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

• image: gcr.io/agones-images/agones-controller:1.8.0-fc5e723
• image: gcr.io/agones-images/agones-sdk:1.8.0-fc5e723
• image: gcr.io/agones-images/agones-ping:1.8.0-fc5e723
• Linux C++ SDK (build): agonessdk-1.8.0-fc5e723-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.8.0-fc5e723.zip

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

• https://fc5e723-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/1735/head:pr_1735 && git checkout pr_1735
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.8.0-fc5e723

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [markmandel]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[google-oss-robot]

New changes are detected. LGTM label has been removed.

[agones-bot]

Build Failed 😱

Build Id: 4a5f233b-571e-4740-b8b5-57e68f52afc0

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• Cloud Build log download

To get permission to view the Cloud Build view, join the agones-discuss Google Group.

[agones-bot]

Build Succeeded 👏

Build Id: 8ea9b18c-e3a2-4dd3-9058-d065d37291dd

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

• image: gcr.io/agones-images/agones-controller:1.8.0-16990ad
• image: gcr.io/agones-images/agones-sdk:1.8.0-16990ad
• image: gcr.io/agones-images/agones-ping:1.8.0-16990ad
• Linux C++ SDK (build): agonessdk-1.8.0-16990ad-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.8.0-16990ad.zip

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

• https://16990ad-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/1735/head:pr_1735 && git checkout pr_1735
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.8.0-16990ad