Add Metrics Port to EventListener service

[khrm]

Metrics port in service is needed by metering serices like prometheus
servicemonitor. It's already there for controller and webhook but not
EventListener which is created.

Changes

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• Includes tests (if functionality changed/added)
• Includes docs (if user facing)
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Release Notes

Service Port for EventListener is accessible as http-metrics

[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-triggers-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/reconciler/eventlistener/eventlistener.go	77.5%	78.0%	0.5

[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-triggers-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/reconciler/eventlistener/eventlistener.go	77.5%	77.5%	0.1

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: savitaashture

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[dibyom]

/lgtm