Which system to choose for application monitoring?

Question

Monitoring

I received the task of optimizing several servers and sites on them. In this matter, I am interested in application monitoring, software management. Approximately the following data: the speed of a single script, resource consumption by the script, how long the database processes its requests, etc. To understand what does not work well, give it to programmers and then just follow. I worked with zabbix as a server status monitor from excellent, but in my opinion it’s not very convenient for monitoring applications. Looked towards newrelic, but too expensive ($150/month!!!). Can you advise something? I would like to cover both applications and the servers themselves with one monitoring server. It is desirable to open-source, but you can also take a subscription for an adequate price. Perhaps tell me some suitable tools/templates for zabbix?