gst on freight in invoice

GST rate would be @ 18% on Freight. (Consider Reverse charge, If applicable)

maximum rate of tax what you given to your GST bill

If the the Freight is shown in the bill in which the 'Metal' is supplied then its consider as composit supply & the rate of gst on freight will be same of the metal (principal goods) GST Rate ( ie 5%)

If you raise the seprate bill freight its exempted as per Notification 12/2017 CT (r)
Note : Not Advisable to Raise seprate Bill for Freight

As per Section 8
[a] a composite supply comprising two or more supplies, one of which is a principal supply, shall be treated as a supply of such principal supply; and
here metal is principal supply and rate of metal is 5 % so 5% is applicable

you have to charge 5% as same as mettal