net/tstun: rate limit "self disco out packet" logging

When this happens, it is incredibly noisy in the logs.
It accounts for about a third of all remaining
"unexpected" log lines from a recent investigation.

It's not clear that we know how to fix this,
we have a functioning workaround,
and we now have a (cheap and efficient) metric for this
that we can use for measurements.

So reduce the logging to approximately once per minute.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

net/tstun/wrap_test.go

@@ -495,7 +495,7 @@ func TestPeerAPIBypass(t *testing.T) {
 func TestFilterDiscoLoop(t *testing.T) {
 	var memLog tstest.MemLogger
 	discoPub := key.DiscoPublicFromRaw32(mem.B([]byte{1: 1, 2: 2, 31: 0}))
-	tw := &Wrapper{logf: memLog.Logf}
+	tw := &Wrapper{logf: memLog.Logf, limitedLogf: memLog.Logf}
 	tw.SetDiscoKey(discoPub)
 	uh := packet.UDP4Header{
 		IP4Header: packet.IP4Header{