fix(ecstore): age LastMinuteLatency samples individually (backlog#806-16)
The rolling one-minute latency window stored bare Duration samples plus a
single, never-updated start_time. Its retain predicate ignored the element
and evaluated the constant now.duration_since(start_time) < 60s, so once the
window had existed for 60s every add() dropped ALL samples and the average
degenerated to the latest single sample.
Each sample now carries its own Instant and is retained by its individual
age. The type backs only in-memory EpHealth + admin display (the wire shape
is target::LatencyStat with curr/avg/max), so Serialize/Deserialize is kept
only to satisfy the enclosing LatencyStat derive; the sample Instant is
serde(skip) and restarts aging on reload.
Zhengchao An
37c1153c1e