How can I find the subset of dataframe that contain elements with Number datatype?

General Usage Performance
2 
julia> using DataFrames

julia> df = DataFrame(col1 = rand(3), col2 = rand(1:10, 3), col3 = rand('a':'c', 3))
10×3 DataFrame
 Row │ a         b      c    
     │ Float64   Int64  Char 
─────┼───────────────────────
   1 │ 0.401624      2  b
   2 │ 0.94679       3  b
   3 │ 0.96397       9  b


julia> names(df, Number)
2-element Vector{String}:
 "col1"
 "col2"
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