Returns the column metadata for one of the caller's own custom Node-RED
tables, with a role inferred from each column's Postgres data type:

• time: always inserted_at, the one column every custom table is
  guaranteed to have. This is the column /timeseries/custom/{table}
  uses for bucketing/filtering, regardless of any other timestamp
  column the flow author configured.
• metric: a numeric column, usable in metrics with any supported
  aggregation.
• dimension: any other column. Not currently usable for querying
  (pivot_on is not yet supported for custom tables).