Transform .csv into 3 columns and a row

id,Product1,Product2,Product3,Product4
1,0.1,0.3,0.8,0.7
2,0.6,0.7,0.5,0.9

id,productname,product_val
1,Product1,0.1
1,Product2,0.3
1,Product3,0.8
1,Product4,0.7
2,Product1,0.6
2.Product2,0.7
3,Product3,0.5

awk -F, 'NR==1 { for (i=1; i1 { for (i=1; i