feat(delivery): add maximum backoff duration

[kahirokunn]

This PR implements the Alpha stage of #9278.

TL;DR: New experimental feature allowing users to opt in to capping normal delivery retry intervals calculated from backoffDelay and backoffPolicy.

Proposed Changes

• 🎁 Add the Alpha delivery-backoff-max feature flag, disabled by default.
• 🎁 Extend DeliverySpec with the optional backoffMax field.
• 🎁 Validate that backoffMax is a positive ISO 8601 duration and reject the field while the feature is disabled.
• 🎁 Propagate backoffMax through Subscription reconciliation and Channel delivery defaults.
• 🎁 Parse backoffMax in RetryConfigFromDeliverySpec() and apply it to linear and exponential backoff calculations.
• 🐛 Prevent linear and exponential backoff calculations from overflowing time.Duration, including when no explicit maximum is configured.
• 🎁 Keep delays requested through Retry-After independently controlled by retryAfterMax.
• 🎁 Add unit and experimental E2E coverage for capped delivery retries.
• 🎁 Update generated API documentation; user-facing documentation is tracked in DeliverySpec.BackoffMax Experimental Feature docs#6686.

Pre-review Checklist

• At least 80% unit test coverage
• E2E tests for any new behavior
• Docs PR for any user-facing impact
• Spec PR for any new API feature (Planned if the experimental feature graduates to Stable.)
• Conformance test for any change to the spec (To be evaluated when the feature graduates to Stable.)

Release Note

The new experimental `delivery-backoff-max` feature flag lets you set `DeliverySpec.backoffMax` to cap normal retry intervals calculated from `backoffDelay` and `backoffPolicy`. The feature is disabled by default.

Docs

📖 knative/docs#6686

[codecov]

Codecov Report

❌ Patch coverage is 89.09091% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.15%. Comparing base (72ec4f4) to head (93bc77a).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/kncloudevents/retries.go	86.04%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9279      +/-   ##
==========================================
+ Coverage   51.06%   51.15%   +0.08%     
==========================================
  Files         411      411              
  Lines       22120    22172      +52     
==========================================
+ Hits        11295    11341      +46     
- Misses       9951     9954       +3     
- Partials      874      877       +3     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[kahirokunn]

/ok-to-test

[dsimansk]

/ok-to-test

shouldn't be needed anymore for you as an org member :)

[kahirokunn]

@dsimansk Thank you!
If you don't mind, could you please review this PR? 🙇‍♂️
I think backoffMax is a very basic tool that offers the essential features any valuable tool should have. 💪

[dsimansk]

@dsimansk Thank you! If you don't mind, could you please review this PR? 🙇‍♂️ I think backoffMax is a very basic tool that offers the essential features any valuable tool should have. 💪

Agreed, I've added one suggestion. Otherwise looks good, given it's feature flag guarded and well test covered.

[kahirokunn]

@dsimansk Thanks for the suggestion, I've addressed it! Could you take another look when you get a chance? 🙏

[kahirokunn]

/test upgrade-tests

[dsimansk]

@Cali0707 or @creydr any change you could chime in please?

[dsimansk]

Proceeding with this as alpha feature.

/approve
/lgtm

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dsimansk, kahirokunn

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Details

Needs approval from an approver in each of these files:

• OWNERS [dsimansk]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment