[fix bug 1094520] Add sampling to sendMetrics

https://bugzilla.mozilla.org/show_bug.cgi?id=1094520

@Osmose, @glogiotatidis r?

We have to get confirmation that the Tableau people are adding the multiplier for the new worksheet. I'm 90% they are, but here is the bug for that confirmation: https://bugzilla.mozilla.org/show_bug.cgi?id=1094516
While ES6 is so great, I'd be a bit more conservative with the syntax here. This will "only" work for FX >= 15 and then the syntax still changes because ES6 is a draft. 

Either do
```
if (!do_samping) {
  do_samping = true;
} 
```
to match how `metric` is done, or change both to something like `b = typeof b !== 'undefined' ?  b : 1;`