struct-with-method.go

import(
  "github.com/mitchellh/mapstructure"
)

type A struct {
  One string
  
  internalA string
}

func (a *A) Metadata() map[string]string {
  return map[string]string{"a": a.internalA}
}

type B struct {
  One *string
  Meta map[string]string `mapstructure:"metadata"`
}

func main() {
  a := A{One: "foo", internalA: "b"}
  
  var b B
  if err := mapstructure.Decode(a, &b); err != nil {
    panic(err)
  }
  
  // b = B{One: &"foo", Meta: map[string]string{"a": "b"}}
  // (Note: ^ I know the above is not valid Go syntax, but it gets
  // the point across)
}