Extend metrics database schema to bridge descriptors
The current metrics database schema contains relay descriptors with all their details and aggregate statistics on bridges, Torperf, and GetTor.
We should extend the schema to contain bridge descriptors with all their details, too, so that we can add new statistics on bridges more easily. Right now, adding new statistics means changing the Java code and parsing all bridge descriptors again. Possible new bridge statistics are number of bridges on hour detail, relay flags, Tor versions, and operating systems of bridges. We could also add bridge pool assignments to the database schema.
(Extending the database schema for Torperf and/or GetTor is probably not as useful: the Torperf data can still be processed without database support, and the raw GetTor data is already aggregated to protect the privacy of GetTor users.)