sockstats: add validation for TCP socket stats

We can use the TCP_CONNECTION_INFO getsockopt() on Darwin to get
OS-collected tx/rx bytes for TCP sockets. Since this API is not available
for UDP sockets (or on Linux/Android), we can't rely on it for actual
stats gathering.

However, we can use it to validate the stats that we collect ourselves
using read/write hooks, so that we can be more confident in them. We
do need additional hooks from the Go standard library (added in
tailscale/go#59) to be able to collect them.

Updates tailscale/corp#9230
Updates #3363

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

net/sockstats/sockstats.go

@@ -42,10 +42,14 @@ const (
 )
 
 type SockStat struct {
-	TxBytes            int64
-	RxBytes            int64
-	TxBytesByInterface map[string]int64
-	RxBytesByInterface map[string]int64
+	TxBytes            uint64
+	RxBytes            uint64
+	TxBytesByInterface map[string]uint64
+	RxBytesByInterface map[string]uint64
+
+	// NOCOMMIT
+	ValidationTxBytes uint64
+	ValidationRxBytes uint64
 }
 
 func WithSockStats(ctx context.Context, label Label) context.Context {