Three small wins from a telemetry audit:
1. Propagation.scores_at/3 wrapped every call in Instrument.span, firing
two handler dispatches that dominated the ~10µs ETS lookup on cache
hits. The map's LiveView fires this on every pan + point-click, so
hot-path latency was mostly telemetry. Span now wraps only the miss
branch (where disk IO makes the duration signal meaningful); hit path
still emits the cheap hit/miss counter the cache-ratio panel reads.
2. Oban queue-depth poller: 10s → 30s. Every replica independently
GROUP-BYs oban_jobs, so each extra replica paid ~3 redundant queries
per minute for a gauge that moves on the hour-scale anyway.
3. Dropped summary(phoenix.endpoint.start.system_time): summarizing a
wall-clock timestamp produces no useful aggregate — stop.duration
below it is the meaningful signal.
