Add timeout delivery metrics

[grac3gao-zz]

Fixes #1522

Proposed Changes

• If the delivery is cancelled because of timeout, report event dispatch time without resp status code. In metric explorer, the dispatch time will look like: (trigger-actor-0 only has timeout event, trigger-actor-1 only has 2xx event)
  

• Add and update some UT

Release Note

🎁 add timeout delivery metrics to broker fanout and retry 

Docs

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: grac3gao

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• OWNERS [grac3gao]

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[yolocs]

/lgtm
/hold

Maybe add Ian for a look.

[knative-test-reporter-robot]

The following jobs failed:

Test name	Triggers	Retries
pull-google-knative-gcp-wi-tests		0/3

Failed non-flaky tests preventing automatic retry of pull-google-knative-gcp-wi-tests:

test/e2e.TestSmokeChannel
test/e2e.TestSmokeCloudPubSubSourceV1beta1
test/e2e.TestSmokeCloudSchedulerSourceV1beta1
test/e2e.TestCloudAuditLogsSourceWithGCPBroker

[grac3gao-zz]

/retest

[yolocs]

/lgtm
/hold
In case @ian-mi has comments

[knative-metrics-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/broker/handler/processors/deliver/processor.go	80.0%	80.2%	0.2
pkg/metrics/delivery_reporter.go	81.8%	82.6%	0.8

[ian-mi]

/lgtm

[ian-mi]

/unhold