I will point out some difficulties with this question here that a proper question must answer. I am quite interested to know the answer myself, of course.

Firstly, it is hard to figure out when some metals appear in a place. Metal working technology is easy to date. I think the distinction is quite imperative. Metals may well have been introduced by means of trade. But large scale production of tools and weapons using a metal is perhaps what is meant by this question.

An equally important question is : what is considered India for this question? Geographic bounds of the republic India? Cultural extent of India has - at various times of the timeline of Indian history - included Afghanistan, Pakistan, Nepal, south east Asia.Of course, by the language of the question OP is limiting the discourse to the time when Vedic tribes were setting up kingdoms in northwest of Indian subcontinent.

Now, in this time, iron is not the predominant metal of tools or weapons. Iron Age in India coincides with Vedic tribes expanding into east of India down the gangetic plains.

At this point, anyone claiming that Vedic tribes introduced metal working to the subcontinent would have a tough task ahead of them. They would have to find archeological evidence of metals appearing widespread corresponding to the appearance of Vedic tribes in India. Which is hard to come by. It may well be that in the time of Janapadas new metals started being used for coinage, tools, weapons etc but those would not be introductions. Those would be considered indigenous developments or external influence.