Respect Retry-After header on 503 responses for RetryClient

[RobertBrunhage]

it is generally recommended that on 503, if the server responds with the Retry-After header we should respect that and if it's not present we should default back exponential backoff.

Is this something that coud be considered to be added directly into the RetryClient, so we have that as default behaviour?

I am asking this because we already retrying 503 by default.

[brianquinlan]

The most obvious way of doing this is to make BaseResponse available in the delay function. But that would be a breaking change.

Can you think of another way of doing this?