Collapse each group to one value per configured name (last writer wins,
config order) so a repeated name can't emit duplicate kula_custom_metric
series and break the scrape. Precompute the name index once instead of
per message. Make debugDone atomic (multi-goroutine read). Add a 5m idle
read deadline and log real read errors (e.g. ErrTooLong). Adds a dedup
unit test and a repeated-name fuzz seed.