ipn/ipnlocal: add IPN Bus NotifyRateLimit watch bit NotifyRateLimit

Limit spamming GUIs with boring updates to once in 3 seconds, unless
the notification is relatively interesting and the GUI should update
immediately.

This is basically @barnstar's #14119 but with the logic moved to be
per-watch-session (since the bit is per session), rather than
globally. And this distinguishes notable Notify messages (such as
state changes) and makes them send immediately.

Updates tailscale/corp#24553

Change-Id: I79cac52cce85280ce351e65e76ea11e107b00b49
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/debug.go

@@ -213,6 +213,7 @@ var debugCmd = &ffcli.Command{
 				fs := newFlagSet("watch-ipn")
 				fs.BoolVar(&watchIPNArgs.netmap, "netmap", true, "include netmap in messages")
 				fs.BoolVar(&watchIPNArgs.initial, "initial", false, "include initial status")
+				fs.BoolVar(&watchIPNArgs.rateLimit, "rate-limit", true, "rate limit messags")
 				fs.BoolVar(&watchIPNArgs.showPrivateKey, "show-private-key", false, "include node private key in printed netmap")
 				fs.IntVar(&watchIPNArgs.count, "count", 0, "exit after printing this many statuses, or 0 to keep going forever")
 				return fs
@@ -500,6 +501,7 @@ var watchIPNArgs struct {
 	netmap         bool
 	initial        bool
 	showPrivateKey bool
+	rateLimit      bool
 	count          int
 }
 
@@ -511,6 +513,9 @@ func runWatchIPN(ctx context.Context, args []string) error {
 	if !watchIPNArgs.showPrivateKey {
 		mask |= ipn.NotifyNoPrivateKeys
 	}
+	if watchIPNArgs.rateLimit {
+		mask |= ipn.NotifyRateLimit
+	}
 	watcher, err := localClient.WatchIPNBus(ctx, mask)
 	if err != nil {
 		return err