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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[danehans]

/ok-to-test

[danehans]

@delavet great job with this PR. Fix the linter issue and all should be good.

/lgtm

[k8s-ci-robot]

New changes are detected. LGTM label has been removed.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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

Sorry for these linter issues. They have been resolved, please help check this again @danehans