cmd/tailscaled: add some more depchecker dep tests

As we look to add github.com/prometheus/client_golang/prometheus to
more parts of the codebase, lock in that we don't use it in tailscaled,
primarily for binary size reasons.

Updates #12614

Change-Id: I03c100d12a05019a22bdc23ce5c4df63d5a03ec6
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/tailscaled_test.go

@@ -31,6 +31,8 @@ func TestDeps(t *testing.T) {
 		BadDeps: map[string]string{
 			"testing":                                        "do not use testing package in production code",
 			"gvisor.dev/gvisor/pkg/hostarch":                 "will crash on non-4K page sizes; see https://github.com/tailscale/tailscale/issues/8658",
+			"google.golang.org/protobuf/proto":               "unexpected",
+			"github.com/prometheus/client_golang/prometheus": "use tailscale.com/metrics in tailscaled",
 		},
 	}.Check(t)
 }