net/connstats: enforce maximum number of connections (#6760)

The Tailscale logging service has a hard limit on the maximum
log message size that can be accepted.
We want to ensure that netlog messages never exceed
this limit otherwise a client cannot transmit logs.

Move the goroutine for periodically dumping netlog messages
from wgengine/netlog to net/connstats.
This allows net/connstats to manage when it dumps messages,
either based on time or by size.

Updates tailscale/corp#8427

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

wgengine/magicsock/magicsock_test.go

@@ -135,7 +135,7 @@ func runDERPAndStun(t *testing.T, logf logger.Logf, l nettype.PacketListener, st
 type magicStack struct {
 	privateKey key.NodePrivate
 	epCh       chan []tailcfg.Endpoint // endpoint updates produced by this peer
-	stats      connstats.Statistics    // per-connection statistics
+	stats      *connstats.Statistics   // per-connection statistics
 	conn       *Conn                   // the magicsock itself
 	tun        *tuntest.ChannelTUN     // TUN device to send/receive packets
 	tsTun      *tstun.Wrapper          // wrapped tun that implements filtering and wgengine hooks
@@ -1053,11 +1053,15 @@ func testTwoDevicePing(t *testing.T, d *devices) {
 		}
 	}
 
-	m1.conn.SetStatistics(&m1.stats)
-	m2.conn.SetStatistics(&m2.stats)
+	m1.stats = connstats.NewStatistics(0, 0, nil)
+	defer m1.stats.Shutdown(context.Background())
+	m1.conn.SetStatistics(m1.stats)
+	m2.stats = connstats.NewStatistics(0, 0, nil)
+	defer m2.stats.Shutdown(context.Background())
+	m2.conn.SetStatistics(m2.stats)
 
 	checkStats := func(t *testing.T, m *magicStack, wantConns []netlogtype.Connection) {
-		_, stats := m.stats.Extract()
+		_, stats := m.stats.TestExtract()
 		for _, conn := range wantConns {
 			if _, ok := stats[conn]; ok {
 				return