If multiple sites were requested in {@link build()} or {@link factory()} the result will
be a DataTable\Map that is indexed by site ID.
If multiple periods were requested in {@link build()} or {@link factory()} the result will
be a {@link DataTable\Map} that is indexed by period.
The site ID index is always first, so if multiple sites & periods were requested, the result
will be a {@link DataTable\Map} indexed by site ID which contains {@link DataTable\Map} instances that are
indexed by period.
_Note: Every DataTable instance returned will have at most one row in it. The contents of each
row will be the requested metrics for the appropriate site and period._