code: func (h *Head) Appender() Appender {
  	h.metrics.activeAppenders.Inc()
  
  	// The head cache might not have a starting point yet. The init appender
  	// picks up the first appended timestamp as the base.
  	if h.MinTime() == math.MaxInt64 {
  		return &initAppender{head: h}
  	}
  	return h.appender()
  }
docs: // Appender returns a new Appender on the database.