Record EPP NormalizedTimePerOutputToken metric on streaming mode

[dharaneeshvrd]

What type of PR is this?

/kind bug
/kind failing-test

What this PR does / why we need it:
Add code changes to record NormalizedTimePerOutputToken metric in EPP, which is expected in e2e epp test.

Which issue(s) this PR fixes:

Fixes #939

Does this PR introduce a user-facing change?:

NONE

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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

/cc @delavet

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[kfswain]

Hey @dharaneeshvrd! Thanks for the PR, do you mind adding this metric to our hermetic tests to validate the behavior?

gateway-api-inference-extension/test/integration/epp/hermetic_test.go

Line 705 in ef666c1

wantMetrics: map[string]string{`inference_objective_input_tokens`: `

[dharaneeshvrd]

@kfswain Updated the hermetic test. PTAL!