struct中缺少函数体和字符串标记

I find some function with no function body in Go. I know it means external function in Go. But where can I find the function boby?

type Creator func(*Beat, *common.Config) (Beater, error)

I also find a string in Go struct. What does it mean?

type BeatConfig struct {
    // output/publishing related configurations
    Output common.ConfigNamespace `config:"output"`
}

All of the above code can be found in elasticsearch beats: https://github.com/elastic/beats/blob/master/libbeat/beat/beat.go

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This:

type Creator func(*Beat, *common.Config) (Beater, error)

Is not a function declaration, it's a type declaration. It creates a new type (with a function type as its underlying type), it does not declare any functions, and so no function body is required, it wouldn't make sense and you cannot provide a body in there.

You can use it for example to create a variable of that type, and store an actual function value which has the same underlying type.

Example:

type Creator func(*Beat, *common.Config) (Beater, error)

func SomeCreator(beat *Beat, config *common.Config) (Beater, error) {
    // Do something
    return nil, nil
}

func main() {
    var mycreator Creator = SomeCreator
    // call it:
    beater, err := mycreator(&Beat{}, &common.Config{})
    // check err, use beater
}

And this field declaration:

Output common.ConfigNamespace `config:"output"`

Contains a tag declaration for the Output field. For more details, see What are the use(s) for tags in Go?

As @icza said, Creator is a type declaration.

In your case, each implementation of Beat has implemented Creator interface. Here is beats's document.