Fix exponential backoff for api.LifetimeWatcher

[andrewstucki]

I just opened up a bug for this PR, this fixes it. Here's the original context from the bug:

Describe the bug
We were using the Vault LifetimeWatcher from the api package in an internal project and noticed an issue with the backoff behavior of token renewal that was causing a bunch of our tests to fail when we upgraded to a new version of Vault.

The bug is here:

vault/api/lifetime_watcher.go

Lines 348 to 354 in 1274f2d

	var sleepDuration time.Duration
	if errorBackoff == nil {
	sleepDuration = r.calculateSleepDuration(remainingLeaseDuration, priorDuration)
	} else if errorBackoff.NextBackOff() == backoff.Stop {
	return err
	}

sleepDuration appears to be the time.Duration used prior to re-running the renewal loop. In the case when errBackoff is nil, then a simple backoff duration is calculated based on the call to calculateSleepDuration. If errBackoff is not nil then sleepDuration is never set and the timeout in the following select block immediately fires again.

In our testing environment this was caught because our mock Vault server was returning an invalid response, so the renew operation was failing and we were getting an inordinate amount of immediate retries.

The fix is just refactoring the above block to capture the errBackoff.NextBackoff() value as sleepDuration.

fixes: #26382

[github-actions]

CI Results:
All Go tests succeeded! ✅

[github-actions]

Build Results:
All builds succeeded! ✅

[peteski22]

Thanks! LGTM.

Please could you add a changelog file before it's merged?

[andrewstucki]

@peteski22 Thanks for the review. Added a changelog entry, not entirely sure of the vault project labeling conventions for changelog entries, so let me know if I should do something other than api. Also, do I need to label this with some sort of milestone label? Also, backports? Not sure if we do backport releases for the subpackage libraries or not.

[ccapurso]

Looks good, thanks for adding the changelog entry!

[andrewstucki]

@ccapurso / @peteski22 : Just FYI, looks like I don't have permission to merge since I'm not part of the Vault org, so think someone from your team will have to. Thanks!

[ccapurso]

I can merge. Thanks again for the fix!