Whenever I needed to do a pivot in postgres, I used this approach that is described in this stackoverflow anwser:

https://stackoverflow.com/questions/20618323/create-a-pivot-...

So for example when you have a table like described (column_name, meta_key, value), you would create a query like this:

  SELECT
    column_name,
    MAX(CASE WHEN meta_key='total_rows' THEN value ELSE NULL END) AS total_rows,
    MAX(CASE WHEN meta_key='not_null_count' THEN value ELSE NULL END) AS not_null_count,
    -- for all other metrics....
  FROM tall_table
  GROUP BY 1

(edit: formatting)