A stats group — one external data source (e.g. "server1"). Owns a
dedicated Barograph SQLite database (see PhoenixKitStats.DatabaseManager)
and a Graphite plaintext ingest port that a collector on that source
writes metrics to.
Tables are created by PhoenixKitStats.Migrations.Schema, not by this
schema module — see migration_module/0 on PhoenixKitStats.