Add Ephemeral Metadata for Blue Green strategy

[cooperbenson-qz]

Summary

Functionality for setting ephemeral metadata for Canary rollouts was proposed in #455 and implemented in #770. I would like to extend this functionality to the BlueGreen strategy as well. This would be done by adding previewMetadata and activeMetadata to the BlueGreen spec and using similar reconciliation logic to apply them to the replicasets and pods.

Use Cases

Same use cases as ephemeral metadata for canary rollouts (i.e. tagging metrics with consistent labels). My specific use case is to tag Prometheus metrics with a label if the pod is active and use that for selecting custom HPA metrics.

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[cooperbenson-qz]

Worth mentioning that I'm planning on authoring a PR for this if there are no objections to the proposed implementation 😄

[jessesuen]

@cooperbenson-qz this sounds great to have parity for blue-green. No objections to the proposal or you working on this. I'll assign the issue to you. Thanks!