Docker's one-shot stats API (stream=false) returns PreCPUStats from the
daemon's internal cache, which many Docker versions don't update between
non-streaming reads. This causes every call to return the same stale
PreCPUStats from container start, producing a constant lifetime-average
CPU% (e.g. 3.4%) instead of current usage.
Switch to always using manual delta tracking, which stores the previous
sample from our own reads and computes accurate deltas between collection
cycles. The first cycle returns 0 while establishing a baseline; all
subsequent cycles produce correct current CPU percentages.
rcourtman
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