Issue 3529 metrics to piggybank

@shane-tomlinson mind taking a look? This is a speculative PR, so opening it against my repo.

This is a proposed fix for mozilla/identity-ops#102, as well as the linked issue mozilla/browserid#3529.

I need to make sure the data format makes sense, but aside from the format of the messages, this seems like it'll solve the problem of getting RP data into kpiggybank, so that it can be aggregated/counted/etc along with our other RP metrics.