DataFrame.describe should return a DataFrame with summary statistics.
If cols is empty, then run describe on all numeric columns.
The returned DataFrame should have 5 rows (count, mean, stddev, min, max) and n + 1 columns. The 1st column is the name of the aggregate function, and the next n columns are the numeric columns of interest in the input DataFrame.
Similar to Pandas (but removing percentile since accurate percentiles are too expensive to compute for Big Data)