add metadata to agones webhook autoscaler request

[swermin]

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
/kind release

What this PR does / Why we need it:
This PR extends the webhook autoscaler with metadata, so that the the service retrieving the request can use the lables/annotations to do the calculation

Which issue(s) this PR fixes:
Closes #3951

Special notes for your reviewer:

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[agones-bot]

Build Failed 😭

Build Id: 5ade9d2e-d20e-4cad-b76b-697960ab7ff0

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[igooch]

Thanks for the contribution. Could you please also add unit and e2e tests?

[gongmax]

@swermin kindly ping on this, could you resolve the conflicts and address the above comments?

[swermin]

Yes ofc! I have unit tests ready and just left with the e2e tests. I’ll push the current changes and focus on the e2e tests today and this weekend if needed!

[agones-bot]

Build Failed 😭

Build Id: 166b3f1b-5297-4b2a-bc9b-3b0ec925abe5

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[agones-bot]

Build Failed 😭

Build Id: 984531ec-7bcc-4fa4-bf36-089d5719a947

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[swermin]

@igooch do you have any good suggestions on how to do the e2e tests for this?
Current default webhook autoscaler is using the example image to test the scaling. I feel like adding support in that image feels a bit weird but I have no good idea on how to add something

[agones-bot]

Build Failed 😭

Build Id: 0ef393db-4a59-403f-8729-4a86e54fb6d3

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