Should I join metallurgy at PEC chandigarh though I have interest in CSE or should I drop an year to make it to IITS/NITS/BITS?

If you're are interested in CSE, then you can choose one of the lower-ranked colleges than IITs and NITs or PEC Chandigarh and settle there with CSE course. As college is partly responsible for good placements and career. It mainly depends on your hard work and efforts and your ability to do well in your academics. 

You can also take a year drop too, if you don't want to settle in a lower-ranked college, and prepare yourself better for JEE exam and clear out the exam with a much better score and ranking to get selected in CSE course in one of the top engineering institutes of our country. 

There is also another option, which is a bit risky and will also require you to do a lot of hard work. This is taking up metallurgy in PEC and then scoring a CGPA of 10 or close to 10 in your first year and then apply for a stream change. But this doesn't guarantee that you will get to change to your preferred stream and you may end up studying metallurgy for the rest of the years. 

These are the option you have, now the final decision is yours.