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The compare-metrics-stats subtest reset two independent counting systems (physical connection counters and expvar.Int user metrics) non-atomically. Background WireGuard keepalives arriving between the resets could increment one system but not the other, causing off-by-one packet/byte mismatches in either direction. Replace the reset-then-compare pattern with snapshot-and-delta: snapshot both systems before pings, snapshot again after, and compare the deltas. This eliminates the non-atomic reset window entirely. As a belt-and-suspenders safety net, tolerate a difference of exactly one packet (and corresponding bytes) from a stray keepalive that could still arrive in the narrow window between the two snapshots. flakestress passes with ~5900 runs (~2800 without -race, ~3100 with -race) but it also passed previously too. This is an annoying one to repro. Fixes #11762 Change-Id: I3447ad67e71c8146e85eed38b7a665033ef9e284 Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>