Function get_retry_after 

pub fn get_retry_after(
    headers: &Headers,
    now: fn() -> OffsetDateTime,
    retry_headers: &[HeaderName],
) -> Option<SignedDuration>

Get the duration to delay between retry attempts, provided by the headers from the response.

This function checks for retry-after headers in the order specified by retry_headers.

If no header is provided, None is returned.