Create a broker spec for metrics collection

We're in the process of creating a module for the collection of snowflake metrics (legacy/trac#21315 (moved), legacy/trac#29461 (moved)). We need a better place to put the spec for the metrics data output by the snowflake broker than a comment in the source code (see https://trac.torproject.org/projects/tor/ticket/29461#comment:5)

A spec for the broker will also be useful to expand upon later to specify how the broker interacts with other pieces of either Snowflake or the Tor ecosystem in the case that the broker assumes more of the responsibilities of BridgeDB.