BUG/MINOR: log: Fix error message when using unavailable fetch in logfmt

BUG/MINOR: log: Fix error message when using unavailable fetch in logfmt

The following configuration:

    defaults
    	unique-id-format TEST-%[srv_name]

    frontend fe_http
    	mode http

    	bind :::8080 v4v6

Emitted the following error:

    [ALERT]    (219835) : Parsing [./patch.cfg:2]: failed to parse unique-id : sample fetch <srv_name]> may not be reliably used here because it needs 'server' which is not available here.

The `]` in the name of the sample fetch should not be there.

This bug exists since at least HAProxy 2.4, which is the oldest supported
version. The fix should be backported there.

Reviewed-by: Volker Dusch <github@wallbash.com>