feat: add trigger sink failures metric

[Sapthagiri777]

Fixes #465

Proposed Changes

• Add TRIGGER_SINK_FAILURES counter metric to Metrics.java
• Increment the counter in WebClientCloudEventSender when all retries are exhausted or on non-retryable failures
• Preserve existing retry behaviour while exposing total sink failures per trigger

Release Note

Docs

No user-facing docs change included. This metric follows the existing eventing metrics conventions.

[knative-prow]

Welcome @Sapthagiri777! It looks like this is your first PR to knative-extensions/eventing-kafka-broker 🎉

[knative-prow]

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[knative-prow]

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[Sapthagiri777]

Hey @Leo6Leo @pierDipi . This is my first PR here - pretty excited about it!
Just added the sink failures metric from #465. Would really appreciate it if someone could run /ok-to-test when you get a chance.