code: func SetDebugInfo(err error, meta metadata.MD) {
  	if _, ok := err.(*trace.TraceErr); !ok {
  		return
  	}
  	out, err := json.Marshal(err)
  	if err != nil {
  		return
  	}
  	meta[DebugReportMetadata] = []string{
  		base64.StdEncoding.EncodeToString(out),
  	}
  }
docs: // SetDebugInfo adds debug metadata about error (traces, original error)
  // to request metadata as encoded property