Argument.make_metavar ignores ParamType.get_metavar ; is not configurable to fix

[sirosen]

If you do the following, you get bad behavior:

class MyType(click.ParamType):
    def get_metavar(self, param):
        return "MY_TYPE"

@click.command("foo")
@click.help_option()
@click.argument("param", type=MyType())
def foo_command(param):
    pass

foo --help produces a metavar of PARAM instead of the more logically correct MY_TYPE.

This is because Argument.make_metavar never looks at self.type.get_metavar(self).

Either this should be fixed so that Argument.make_metavar uses the type metavar by default, or Argument.make_metavar should take a parameter that lets you toggle between the default "name" metavar and the type's default metavar.