onRetryAttempt is not awaited / custom back off strategy

[simllll]

Even though in the documentation it's stated that onRetryAttempt can return a promise,
the promise is not awaited:
see

gaxios/src/common.ts

Line 147 in bf6afeb

onRetryAttempt?: (err: GaxiosError) => Promise<void> | void;

and

gaxios/src/retry.ts

Line 79 in bf6afeb

config.onRetryAttempt(err);

My challenge is to build a custom retry handler, which checks for retry-after headers and so on..
I see that adding await would maybe change some behaviour for end-users.. so I would instead vote for a property that allows to override the default expoenitial back off handler? Would that be an option? I can prepare a PR in this case.

[simllll]

Bascially it would be just here:

gaxios/src/retry.ts

Line 73 in bf6afeb

const backoff = new Promise(resolve => {

to chagne this line to:

const backoff = config.backoffHandler && config.backoffHandler(err) ||  new Promise(resolve => {
    setTimeout(resolve, delay);
  });

and backoffHandler is an optional function that returns a Promise

[bcoe]

@simllll thank you for the bug report.

[chandlervdw]

@simllll would this allow for utilizing an exponential backoff retry strategy?

[simllll]

Yes, in this case you can do whatever you want to do... Including exponential back off

[bcoe]

@simllll switched to feature request from bug, since this is requesting an addition to functionality.

Would you be interested in submitting a patch for this functionality?