Add "monetized google searches" as new default metric for Fx desktop & mobile

[danielkberry]

Our current all/tagged searches metrics don't capture shifts in search behavior from google to other providers, which we want to watch for. Here's an example of how this was done for a past experiment

[rebecca-burwei]

Revenue team has approved this methodology for Desktop and Mobile. It's not perfect, but we think it's accurate enough. One caution is that the UDF may need regular maintenance to keep the business logic and partner codes up-to-date and there is no plan for that -- a plan would require a commitment from Finance or BD.

A pre-emptive caution: Don't use the UDF for partners other than Google and Bing. The business logic is incomplete.