Add a user-defined callback for state changes

[nabeken]

I'm planning to migrate from https://github.com/sony/gobreaker to this implementation since this library has several features I really want to use it.

Only the missing feature here is a user-defined callback for state changes. I found #9 but I don't see new updates since the last year.

Here is my attempt to implement the feature. Hope it sounds good enough.

Thanks for the great library.

[matope]

@nabeken Thank you for your contribution and sorry to be late!

LGTM!

The only my concern was that the hook is invoked during lock. It might lead some performance issues because nobody can't Do() on the CB during lock.

However, If we somehow let the hook run out of lock (maybe using a goroutine), the user need to introduce complexity of concurrency. For example, your test should be like below. Those additional concurrency primitives looks totally painful.

// ....
var mu sync.Mutex
var wg sync.WaitGroup
wg.Add(len(expectedStateChanges))
hook := func(from, to circuitbreaker.State) {
		mu.Lock()
		defer mu.Unlock()
		defer wg.Done()
		actualStateChanges = append(actualStateChanges, stateChange{
			from: from,
			to:   to,
		})
}

// some mutations...

wg.Wait()

All right. Let's run the hook function synchronously during lock. Instead, it is the user's responsibility to keep the function thin.

Thank you for your great PR!

[nabeken]

@matope Thanks for the merge. I think calling the hook within the lock is reasonable guarantee which the library provides. The user can do whatever they wants if the performance matters (e.g. deferring to goroutines as you mention).