system traces: stamp one timestamp per poll, not per value #57

Within one ProcessStatsTracker poll each emitted value is stamped separately, so values from one reading can differ by a few microseconds/ms. Viewer correlation across traces from the same poll is cleaner with a single timestamp per poll.

Fix: take the timestamp once per poll and pass it through the sink.

Deferred from PR #55 (trace producers over shared trackers).

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