[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
Do we want `metric` to be an optional argument? I'd vote against it to avoid skipping setting a value by mistake, but I don't have strong feelings about this.