onRetry callback of some description

[freman]

Is your feature request related to a problem? Please describe.
We'd like to track when the library is performing retries

Describe the solution you'd like
Perhaps an onRetry callback, perhaps something like
func (c *Client) onRetry(func (req, resp) error)

We can use that to record metrics and even optionally discard a request by returning error?

Describe alternatives you've considered
Hijacking stderr to read the log.Printf output...