Support retriable func condition

[tamayika]

Closes #686

Changes

Add retry condition on arbitratry error.
This option overcedes WithCodes and default retriable logic.

Verification

Test code added.

[ggilmore]

This is a drive-by comment, but I think your approach would be better if you:

1. Have your type RetriableFunc func(err error) bool be the only option to be able to accept / reject errors (e.g. get rid of the status codes slice) from the retry options struct.

Ex:

type options struct {
	max             uint
	perCallTimeout  time.Duration
	includeHeader   bool
-	codes           []codes.Code
	backoffFunc     BackoffFunc
	onRetryCallback OnRetryCallback
+	retriableFunc   RetriableFunc
}

2. Refactor the WithCodes options func as a retriableFunc:

// WithCodes sets which codes should be retried.
//
// Please *use with care*, as you may be retrying non-idempotent calls.
//
// You cannot automatically retry on Cancelled and Deadline, please use `WithPerRetryTimeout` for these.
func WithCodes(retryCodes ...codes.Code) CallOption {
	return CallOption{applyFunc: func(o *options) {
                o.retriableFunc = func(err error) bool {
                     c := status.Code(err)

                     for _, code := range retryCodes {
                        if c == code {
                           return true 
                        }
                     }
                     
                     return false
                 }
	}}
}

This maintains backwards compatibility with the existing of the library that are using WithCodes, and gives a library a deeper interface (fewer APIs needed to achieve the same functionality).

[johanbrandhorst]

Hi, thanks for your PR! This is nice, but I also agree with @ggilmore that we could simplify the internal logic to use the new function. Mind making the changes suggested?

[tamayika]

@johanbrandhorst @ggilmore
thanks, I refactored WithCodes option.

[johanbrandhorst]

LGTM

[johanbrandhorst]

Thanks for your contribution!