feat(metrics): Migrate from OpenCensus to OpenTelemetry

[anithapriyanatarajan]

Fixes issue: #1355

Changes

This PR Migrates Chains metrics implementation from the deprecated OpenCensus library to OpenTelemetry to align with Tekton Pipeline and Knative's observability stack modernization.

This migration is necessary because:

• Knative deprecated knative.dev/pkg/metrics (OpenCensus-based) and removed it from the latest versions
• Knative introduced knative.dev/pkg/observability with OpenTelemetry support as the replacement metrics framework
• Tekton Pipeline is migrating to OpenTelemetry (PR #9043)
• OpenCensus is no longer maintained and the ecosystem has standardize on OpenTelemetry

Changes include:

• Update knative.dev/pkg to v0.0.0-20260120122510-4a022ed9999a (includes new observability package with OpenTelemetry support)
• Update tektoncd/pipeline dependency to PR #9043 commit (f8f832f42539) for OpenTelemetry compatibility
• Replace deprecated knative.dev/pkg/metrics imports with direct OpenTelemetry (go.opentelemetry.io/otel) in pkg/pipelinerunmetrics and pkg/taskrunmetrics
• Convert OpenCensus stats.Float64Measure and view.View to OpenTelemetry Int64Counter instruments
• Update metric recording from metrics.Record() with tags to OTel's Counter.Add() with attributes
• Simplify tests to work without deprecated knative.dev/pkg/metrics/metricstest

The metrics API exposed by Chains remains unchanged - this is an internal implementation migration. All existing metrics (pipelinerun_sign_created_total,taskrun_payload_uploaded_total, etc.) continue to be exported with the same
names and labels.

Relates to tektoncd/pipeline#9043
Relates to knative/serving observability migration: https://knative.dev/docs/serving/observability/metrics/serving-metrics/

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has Docs included if any changes are user facing
• Has Tests included if any functionality added or changed
• Follows the commit message standard
• Meets the Tekton contributor standards (including
  functionality, content, code)
• Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings)
• Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

 Migrated from deprecated OpenCensus to OpenTelemetry for metrics collection due to Knative's deprecation of their OpenCensus-based metrics package.

/kind feature

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[tekton-robot]

@anithapriyanatarajan: PR needs rebase.

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