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Fix three independent flake sources, at least as debugged by Claude, though empirically no longer flaking as it was before: 1. Poll for connection counter data instead of reading immediately. The conncount callback fires asynchronously on received WireGuard traffic, so after counts.Reset() there is no guarantee the counter has been repopulated before checkStats reads it. Use tstest.WaitFor with a 5s timeout to retry until a matching connection appears. 2. Replace the *2 symmetry assumption in global metric assertions. metricSendUDP and friends are AggregateCounters that sum per-conn expvars from both magicsock instances. The old assertion assumed both instances had identical packet counts, which breaks under asymmetric background WireGuard activity (handshake retries, etc). The new assertGlobalMetricsMatchPerConn computes the actual sum of both conns' expvars and compares against the AggregateCounter value. 3. Tolerate physical stats being 0 when user metrics are non-zero. A rebind event replaces the socket mid-measurement, resetting the physical connection counter while user metrics still reflect packets processed before the rebind. Log instead of failing in this case. Also move counts.Reset() after metric reads and reorder the reset sequence (counts before metrics) to minimize the race window. Fixes tailscale/tailscale#13420 Change-Id: I7b090a4dc229a862c1a52161b3f2547ec1d1f23f Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>