net/connstats: prepare to remove package (#17554)

The connstats package was an unnecessary layer of indirection.
It was seperated out of wgengine/netlog so that net/tstun and
wgengine/magicsock wouldn't need a depenedency on the concrete
implementation of network flow logging.

Instead, we simply register a callback for counting connections.
This PR does the bare minimum work to prepare tstun and magicsock
to only care about that callback.

A future PR will delete connstats and merge it into netlog.

Updates tailscale/corp#33352

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

wgengine/magicsock/derp.go

@@ -717,8 +717,8 @@ func (c *Conn) processDERPReadResult(dm derpReadResult, b []byte) (n int, ep *en
 	}
 
 	ep.noteRecvActivity(srcAddr, mono.Now())
-	if stats := c.stats.Load(); stats != nil {
-		stats.UpdateRxPhysical(ep.nodeAddr, srcAddr.ap, 1, dm.n)
+	if update := c.connCounter.Load(); update != nil {
+		update(0, netip.AddrPortFrom(ep.nodeAddr, 0), srcAddr.ap, 1, dm.n, true)
 	}
 
 	c.metrics.inboundPacketsDERPTotal.Add(1)