add e2e test for epp metrics

[delavet]

This PR is related to #915 and adds an e2e test case to verify whether the epp exposes the correct metrics after traffic is generated.
This e2e test actually revealed an issue: currently the normalized_time_per_output_token_seconds metric is not being recorded correctly. However, to maintain the atomicity of this PR, I have left a TODO and will consider submitting a new issue to track the problem I discovered.

[k8s-ci-robot]

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This pull-request has been approved by: delavet
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[k8s-ci-robot]

Hi @delavet. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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