code: func (h Headers) ForeachKey(handler func(key, val string) error) error {
  	for k, v := range h {
  		// Skip any non string values
  		stringValue, ok := v.(string)
  		if !ok {
  			continue
  		}
  
  		if err := handler(k, stringValue); err != nil {
  			return err
  		}
  	}
  
  	return nil
  }
docs: // ForeachKey on Headers implements opentracing.TextMapReader for trace propagation.
  // It is essentially the same as the opentracing.TextMapReader implementation except
  // for the added casting from interface{} to string.