cmd/derpprobe: allow running all probes at the same time

This allows disabling spread mode, which is helpful if you are manually
running derpprobe in `--once` mode against a small number of DERP
machines.

Updates https://github.com/tailscale/corp/issues/9916

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

cmd/derpprobe/derpprobe.go

@@ -23,13 +23,14 @@ var (
 	derpMapURL = flag.String("derp-map", "https://login.tailscale.com/derpmap/default", "URL to DERP map (https:// or file://)")
 	listen     = flag.String("listen", ":8030", "HTTP listen address")
 	probeOnce  = flag.Bool("once", false, "probe once and print results, then exit; ignores the listen flag")
+	spread     = flag.Bool("spread", true, "whether to spread probing over time")
 	interval   = flag.Duration("interval", 15*time.Second, "probe interval")
 )
 
 func main() {
 	flag.Parse()
 
-	p := prober.New().WithSpread(true).WithOnce(*probeOnce)
+	p := prober.New().WithSpread(*spread).WithOnce(*probeOnce)
 	dp, err := prober.DERP(p, *derpMapURL, *interval, *interval, *interval)
 	if err != nil {
 		log.Fatal(err)