This allows to use the `unique-id` fetch within `tcp-check` or `http-check`
ruleset. The format is taken from the checked server's backend (which is
naturally inherited from the corresponding `defaults` section).
This is particularly useful with
http-check send ... hdr request-id %[unique-id]
to ensure all requests sent by HAProxy have a unique ID header attached.
This resolves GitHub Issue #3307.
Reviewed-by: Volker Dusch <github@wallbash.com>
ed0c51d2c0
