net/portmapper: log upnp information

This logs some basic statistics for UPnP, so that tailscale can better understand what routers
are being used and how to connect to them.

Signed-off-by: julianknodt <julianknodt@gmail.com>

net/portmapper/portmapper.go

@@ -714,7 +714,10 @@ func (c *Client) Probe(ctx context.Context) (res ProbeResult, err error) {
 				res.UPnP = true
 				c.mu.Lock()
 				c.uPnPSawTime = time.Now()
-				c.uPnPMeta = meta
+				if c.uPnPMeta != meta {
+					c.logf("UPnP meta changed: %+v", meta)
+					c.uPnPMeta = meta
+				}
 				c.mu.Unlock()
 			}
 		case c.pxpPort(): // same value for PMP and PCP