cmd/tailscale/cli: create netmon in debug ts2021

Otherwise we'll see a panic if we hit the dnsfallback code and try to
call NewDialer with a nil NetMon.

Updates #14161

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I81c6e72376599b341cb58c37134c2a948b97cf5f

cmd/tailscale/cli/debug.go

@@ -36,6 +36,7 @@ import (
 	"tailscale.com/hostinfo"
 	"tailscale.com/internal/noiseconn"
 	"tailscale.com/ipn"
+	"tailscale.com/net/netmon"
 	"tailscale.com/net/tsaddr"
 	"tailscale.com/net/tshttpproxy"
 	"tailscale.com/paths"
@@ -850,6 +851,11 @@ func runTS2021(ctx context.Context, args []string) error {
 		logf = log.Printf
 	}
 
+	netMon, err := netmon.New(logger.WithPrefix(logf, "netmon: "))
+	if err != nil {
+		return fmt.Errorf("creating netmon: %w", err)
+	}
+
 	noiseDialer := &controlhttp.Dialer{
 		Hostname:        ts2021Args.host,
 		HTTPPort:        "80",
@@ -859,6 +865,7 @@ func runTS2021(ctx context.Context, args []string) error {
 		ProtocolVersion: uint16(ts2021Args.version),
 		Dialer:          dialFunc,
 		Logf:            logf,
+		NetMon:          netMon,
 	}
 	const tries = 2
 	for i := range tries {

control/controlhttp/constants.go

@@ -76,6 +76,8 @@ type Dialer struct {
 	// dropped.
 	Logf logger.Logf
 
+	// NetMon is the [netmon.Monitor] to use for this Dialer. It must be
+	// non-nil.
 	NetMon *netmon.Monitor
 
 	// HealthTracker, if non-nil, is the health tracker to use.