Each peer runs a WGWatcher that polls GetStats(), and every call dumps
the whole device, so with N peers the watchers perform O(N) full dumps
per poll cycle (O(N^2) work) while each keeps only its own peer's entry.
Wrap the kernel and userspace configurer GetStats() in a short-TTL cache
with singleflight: the staggered per-peer calls share a single device
dump per window and concurrent misses collapse into one dump. The kernel
and userspace WireGuard APIs have no per-peer stats query (a get always
returns the whole device), so a shared cached snapshot avoids the
repeated full dumps.
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