Adding onNext in RetryListener

[MFomichev]

I would like to have the opportunity to listen an event before each retry (including the first one) would be happend in order to add some information to MDC (retry number). I can do it by onError, however that does not represent the actual scenario.

If you do not mind about this functionality, then I will be happy to provide PR.

[dsyer]

You could just use the existing methods for this, I guess, but you just want a neater way to do it as a one liner?