retryAfterHeaderMs(header, now?, max?): number | null
A Retry-After header as milliseconds: delta-seconds (a whole non-negative
number) or an HTTP-date, bounded to max; null for a missing or malformed
header, so the caller falls back to its own backoff.
A Retry-After header as milliseconds: delta-seconds (a whole non-negative number) or an HTTP-date, bounded to
max; null for a missing or malformed header, so the caller falls back to its own backoff.