Drop use of knative.dev/pkg/metrics

[dprotaso]

As part of Knative's migration to OpenTelemetry (as discussed here) we dropped our OpenCensus packages in knative.dev/pkg in the main branch see: knative/pkg#3227

This issue is for Tekton to move off of this package since the next knative.dev/pkg bump will no longer work.

Ideally you'd move to OpenTelemetry - or fork the package.

[twoGiants]

[waveywaves]

We will be moving to OpenTelemetry 🥳
cc @khrm

[khrm]

We had an older pr. I just raised a new one.

[khrm]

BTW, we had a discussion with our team. This is causing a breaking change. We should probably mention in lts breaking change notice. 1.6 and do this in 1.7