ipn/localapi, cmd/tailscale: add CPU & memory profile support, debug command

This was already possible on Linux if you ran tailscaled with --debug
(which runs net/http/pprof), but it requires the user have the Go
toolchain around.

Also, it wasn't possible on macOS, as there's no way to run the IPNExtension
with a debug server (it doesn't run tailscaled).

And on Windows it's super tedious: beyond what users want to do or
what we want to explain.

Instead, put it in "tailscale debug" so it works and works the same on
all platforms. Then we can ask users to run it when we're debugging something
and they can email us the output files.

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

client/tailscale/tailscale.go

@@ -154,6 +154,18 @@ func Goroutines(ctx context.Context) ([]byte, error) {
 	return get200(ctx, "/localapi/v0/goroutines")
 }
 
+// Profile returns a pprof profile of the Tailscale daemon.
+func Profile(ctx context.Context, pprofType string, sec int) ([]byte, error) {
+	var secArg string
+	if sec < 0 || sec > 300 {
+		return nil, errors.New("duration out of range")
+	}
+	if sec != 0 || pprofType == "profile" {
+		secArg = fmt.Sprint(sec)
+	}
+	return get200(ctx, fmt.Sprintf("/localapi/v0/profile?name=%s&seconds=%v", url.QueryEscape(pprofType), secArg))
+}
+
 // BugReport logs and returns a log marker that can be shared by the user with support.
 func BugReport(ctx context.Context, note string) (string, error) {
 	body, err := send(ctx, "POST", "/localapi/v0/bugreport?note="+url.QueryEscape(note), 200, nil)