fix exponential backoff bug in RetryConfigFromDeliverySpec()

[travis-minke-sap]

Fixes #4113

Proposed Changes

Rework the exponential backoff calculation to match the advertised algorithm.

Release Note

The default exponential backoff duration now matches the advertised algorithm in the DeliverySpec.BackoffDelay comments.  User action required to evaluate if DeliverySpec.BackoffDelay settings in `Subscription.spec.delivery` remain appropriate.

[knative-prow-robot]

Welcome @travis-minke-sap! It looks like this is your first PR to knative/eventing 🎉

[knative-prow-robot]

Hi @travis-minke-sap. Thanks for your PR.

I'm waiting for a knative member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[mattmoor]

Produced via:
gofmt -s -w $(find -path './vendor' -prune -o -path './third_party' -prune -o -name '*.pb.go' -prune -o -type f -name '*.go' -print)
goimports -w $(find -name '*.go' | grep -v vendor | grep -v third_party | grep -v .pb.go | grep -v wire_gen.go)

[pierDipi]

Can you backport to 0.17?

/ok-to-test
/lgtm

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: pierDipi, travis-minke-sap

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

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [pierDipi]

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

[travis-minke-sap]

Can you backport to 0.17?

/ok-to-test
/lgtm

Sure - happy to backport it ; ). This is my first knative-eventing PR (outside all the sandbox work) - do I just create another PR against the latest release-0.17 branch or is there more to it ?

[knative-metrics-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/kncloudevents/message_sender.go	Do not exist	44.1%

[pierDipi]

Yes, just another PR against the release-0.17 branch, similar to this: #4114 (which is related to this PR)

[knative-prow-robot]

@travis-minke-sap: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
pull-knative-eventing-go-coverage	f470e26	link	/test pull-knative-eventing-go-coverage

Full PR test history. Your PR dashboard.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository. I understand the commands that are listed here.