What data can nginx collect that can be useful from a marketing point of view?

Question

Nginx

Here is a question. What can be collected at the nginx level, which can be useful for your own analytics? I generally have a Node.js backend, but it seems to me that some things are simply more efficient to do with the final web server and not load the Node.js application with this task as well.
A specific use case, for example, to identify users. User agent, Referer, maybe something else is possible? Well, i.e. it's not about the data that can be collected on the client by executing JS there, but exactly what nginx "sees".
For example, how to determine the IP address? In what format are the logs I need stored in nginx (in Google I find only a description of error logs, etc.)? How then to give it all in an understandable form to my front-end?
Too many questions turned out, I will be grateful for any tips.

Answer 1

Any variables available in the http section ( from Apache with translation to the nginx syntax + their own ), you can also attach modules and lua scripts. You can transfer via proxy_set_header X-Custom-Header-Name $foo.

Answer 2

I would recommend approaching not from the point of view of "what can come in handy", but first to determine exactly what exactly is required for the metrics.
For example, if you have an online store, then you need to manage gross sales and profit in it.
Sales, in turn, depend on how customers pass the so-called. "sales funnel", i.e. steps on the site leading to the purchase.
Seeing how many customers fall off at which step, you can figure out how to improve the performance of this step.
If you look from this point of view, then you first need to collect customer identification + their movement along the steps of the funnel.
Here also think what data for this purpose are necessary.