code: func (m *MultiFormatter) Format(w io.Writer, entry zapcore.Entry, fields []zapcore.Field) {
  	m.mutex.RLock()
  	for i := range m.formatters {
  		m.formatters[i].Format(w, entry, fields)
  	}
  	m.mutex.RUnlock()
  }
docs: // Format iterates over its delegates to format a log record to the provided
  // buffer.