I have:

df = pl.DataFrame({'key':['a','a','a','b','b','b'],'a':[2,4,6,1,2,3]})
print(df)
shape: (6, 2)
┌─────┬─────┐
│ key ┆ a   │
│ --- ┆ --- │
│ str ┆ i64 │
╞═════╪═════╡
│ a   ┆ 2   │
│ a   ┆ 4   │
│ a   ┆ 6   │
│ b   ┆ 1   │
│ b   ┆ 2   │
│ b   ┆ 3   │
└─────┴─────┘

I want to add a column with increasing integers within each group defined by key

df = pl.DataFrame({'key':['a','a','a','b','b','b'],'a':[2,4,6,1,2,3], 'r': [1,2,3,1,2,3]})
print(df)
shape: (6, 3)
┌─────┬─────┬─────┐
│ key ┆ a   ┆ r   │
│ --- ┆ --- ┆ --- │
│ str ┆ i64 ┆ i64 │
╞═════╪═════╪═════╡
│ a   ┆ 2   ┆ 1   │
│ a   ┆ 4   ┆ 2   │
│ a   ┆ 6   ┆ 3   │
│ b   ┆ 1   ┆ 1   │
│ b   ┆ 2   ┆ 2   │
│ b   ┆ 3   ┆ 3   │
└─────┴─────┴─────┘

How do I do it?