Having used MTR on my mac and on Linux system I was always frustrated with the lack of this option from the Junos CLI.  You have always been able to start a shell and run MTR as below:-

nick@ScoobyDoo> start shell
% mtr 8.8.8.8


Although it’s super handy, the inconvenience of starting the shell can be a PITA.

I thought i would have to live with this until I found the following:-

nick@ScoobyDoo> traceroute monitor 8.8.8.8


Who knew!!!!