sockstats: export as client metrics

Though not fine-grained enough to be useful for detailed analysis, we
might as well export that we gather as client metrics too, since we have
an upload/analysis pipeline for them.

clientmetric.Metric.Add is an atomic add, so it's pretty cheap to also
do per-packet.

Updates tailscale/corp#9230
Updates #3363

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