Assuming there are logs generated.
You could use Logstash to consume your logs and push into Elastic Search.
Logstash dynamically ingests, transforms, and ships your data
regardless of format or complexity. Derive structure from unstructured
data with grok, decipher geo coordinates from IP addresses, anonymize
or exclude sensitive fields, and ease overall processing.
https://www.elastic.co/logstash
From Elastic Search, you have a lot of flexibility in how you aggregate data to figure our how many are using your function
https://www.elastic.co/
You can look into Kibana for visual representation of such data as well.
====== Edited ==========
In its simplest form, you will be able to find out:
- Project A is calling functionA()
- Project B is calling functionA(), and functionB()
You will not be able to tell which User, unless you are logging such information.
This all depends on what you are logging, what logstash is going to be parsing and pushing into Elastic Search.