Now we have a Zabbix server running:
https://csa-zabbix-1320158900.us-east-1.elb.amazonaws.com
A complete monitoring service(Like ServerDensity and DataDog) costs too much(around ~300 dollars), and a Zabbix instance cost ~7 dollars by month. Also, we can extend Zabbix with plugins/scripts(Zabbix is under GPL) and we don’t need to request for a support team 
All our basic requirements already are available in Zabbix. If the connection with zabbix-server is interrupted, zabbix-agent will start saving data into a local SQLite database.