And in English the most common last name is "Smith". Both of deal with metals is that a coincidence?

In short, yes:

https://en.wikipedia.org/wiki/Kim_(Korean_name)

In my experience, East Asian surnames tend to indicate the geographic origin of your ancestors, rather than their professions. In the case of the Kims, chances are you had an ancestor in Gimhae. (OTOH, I wouldn't be terribly surprised to learn that Gimhae was once famous for its blacksmiths or goldsmiths.)