Use metric column as prefix

If multiple value columns are returned and a metric column is
returned aswell the metric column will be used as prefix for
the series name

docs/sources/features/datasources/postgres.md

@@ -101,7 +101,7 @@ The resulting table panel:
 
 If you set `Format as` to `Time series`, for use in Graph panel for example, then the query must return a column named `time` that returns either a sql datetime or any numeric datatype representing unix epoch.
 Any column except `time` and `metric` is treated as a value column.
-You may return a column named `metric` that is used as metric name for the value column.
+You may return a column named `metric` that is used as metric name for the value column. If you return multiple value columns and a column named `metric` then this column is used as prefix for the series name.
 
 **Example with `metric` column:**