Experiment authors: please update `testpilot-metrics`

I’ve just published version 2.1.2 of the testpilot-metrics library to npm.

This is a patch update (from 2.1.0) with a fix for a critical bug: GA was counting all pings as a single user. No code changes are needed, but you’ll need to re-run npm install, rebuild, sign, and release your addon to pick up the fix.

If you have questions or comments, feel free to ping me here, in the #testpilot IRC channel, or by email at 6a68 AT mozilla DOT com.