Add a callback hook when throttled

[dormeiri]

I would like to observe every time I reach the throttling limit and get delayed.

I think I can achieve it by calling a callback function when getDelay() is greater than zero here:

p-throttle/index.js

Line 83 in 3dffe70

timeoutId = setTimeout(execute, getDelay());

The callback function can be passed as one of the options in:

p-throttle/index.js

Line 8 in 3dffe70

export default function pThrottle({limit, interval, strict}) {

I suggest something like this: onDelay: (info: { delay: number, queueSize: number }) => void | Promise<void>, WDYT?

I can open a PR if it helps.

Edit: I can live without passing the info to the callback, so this also works for my use-case: onDelay: () => void | Promise<void>

[sindresorhus]

What's the use-case?

[dormeiri]

I want to send a metric when delayed to analyze the performance of my app

I opened a suggestion PR just in case it is OK by you to add this functionality :)
#47