title: Missing function body and string tag in struct
body: <p>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? </p>
  
  <pre><code>type Creator func(*Beat, *common.Config) (Beater, error)
  </code></pre>
  
  <p>I also find a string in Go struct. What does it mean?</p>
  
  <pre><code>type BeatConfig struct {
      // output/publishing related configurations
      Output common.ConfigNamespace `config:"output"`
  }
  </code></pre>
  
  <p>All of the above code can be found in elasticsearch beats:
  <a href="https://github.com/elastic/beats/blob/master/libbeat/beat/beat.go" rel="nofollow noreferrer">https://github.com/elastic/beats/blob/master/libbeat/beat/beat.go</a></p>