I came across this in my search to have a greater understanding of application performance. After having read a few articles I feel this is the “monitoring” of the future. Directly applicable to multiple technologies including php and JavaScript.
The above link is observability instrumentation for the javascropt ecosystem. However, instrumentation for many other environments including php.
Splunk provides a free ebook explaining observability.
https://www.splunk.com/en_us/form/beginners-guide-to-observability.html