MINOR: sample: extend the "when" converter to support an ACL

MINOR: sample: extend the "when" converter to support an ACL

Sometimes conditions to decide of an anomaly are not as easy to define
as just an error or a success. One example use case would be to monitor
the transfer time and fix a threshold.

An idea suggested by Tristan would be to make permit the "when"
converter to refer to a more variable or dynamic condition.

Here we make this possible by making "when" rely on a named ACL. The
ACL then needs to be specified in either the proxy or the defaults
section. Since it is evaluated inline, it may even refer to information
available at the end (at log time) such as the data transfer time. If
the ACL evalutates to true, the converter passes the data.

Example: log "dbg={-}" when fine, or "dbg={... debug info ...}" on slow
transfers:

  acl slow_xfer res.timer.data ge 10000   # more than 10s is slow
  log-format "$HAPROXY_HTTP_LOG_FMT                                \
              fsdbg={%[fs.debug_str,when(acl,slow_xfer)]}          \
              bsdbg={%[bs.debug_str,when(acl,slow_xfer)]}"