test: add e2e test for OpenCensus to OpenTelemetry metrics migration

[khrm]

Adds TestOTelMetrics, a consolidated e2e test for the OC→OTel metrics migration in Triggers (PR #1934). The test creates minimal Triggers resources (TriggerTemplate with a TaskRun, TriggerBinding, EventListener) and scrapes the controller /metrics endpoint on port 9000 to assert:

• All controller gauge metrics are registered (eventlistener_count, triggerbinding_count, clustertriggerbinding_count, triggertemplate_count, clusterinterceptor_count)
• Knative workqueue metrics use the new kn_workqueue_* prefix
• Standard go_* runtime metrics are present
• Old OpenCensus metric names (tekton_triggers_controller_workqueue_*) are absent

Uses expfmt/dto.MetricFamily for proper Prometheus text parsing against the controller pod's metrics endpoint (port 9000, labeled app.kubernetes.io/part-of=tekton-triggers).

Relates to #1934

Changes

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Release Notes

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[Copilot]

Pull request overview

Adds an end-to-end test to validate the OpenCensus → OpenTelemetry metrics migration for Tekton Triggers by scraping the controller’s /metrics endpoint and asserting expected metric family presence/prefixes and absence of legacy OpenCensus names.

Changes:

• Introduces a new TestOTelMetrics e2e test that creates minimal Triggers resources (TriggerTemplate/TriggerBinding/EventListener).
• Scrapes and parses Prometheus text metrics from the controller (port 9000) via the Kubernetes API proxy and asserts expected metric naming/presence.
• Adds explicit checks that legacy OpenCensus workqueue metric names are absent.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 31.92%. Comparing base (511e6f6) to head (3ff6e30).

Additional details and impacted files

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  Coverage   31.92%   31.92%           
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  Files         260      260           
  Lines       11966    11966           
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  Hits         3820     3820           
  Misses       7827     7827           
  Partials      319      319           

Flag	Coverage Δ
unit-tests	31.92% <ø> (ø)

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