code: func interpolationFuncTimestamp() ast.Function {
  	return ast.Function{
  		ArgTypes:   []ast.Type{},
  		ReturnType: ast.TypeString,
  		Callback: func(args []interface{}) (interface{}, error) {
  			return time.Now().UTC().Format(time.RFC3339), nil
  		},
  	}
  }
docs: // interpolationFuncTimestamp