A stateless one-shot CLI can't track a running total across invocations,
so cumulative-by-default semantics required a caller to already know and
report the accumulated value every time -- awkward for the common case
(an agent incrementing a counter by 1 per event). Delta is now the
default: send the contribution since the last report, and the collector
accumulates. --temporality cumulative is available for the rare caller
that already tracks its own running total. gauge behavior unchanged.
