class Faraday::Retry::Middleware

def calculate_retry_after(env)

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Retry-After
MDN spec for Retry-After header:
def calculate_retry_after(env)
  retry_header = @options.rate_limit_retry_header || 'Retry-After'
  parse_retry_header(env, retry_header)
end