Add status code to net/http metrics

[nabokihms]

Hello! I have a minor request for improvement for one of the instrumentation packages.

My use-case:
There is a service, and I want to know the percentage of failed requests on it. Previously there was the combination of httpsnoop and Prometheus client_go to capture status code and add it to metrics as a label.

The net/http handler wrapper already extracts status code from the response writer but adds it only to span. See here. And there is a useful labeler, but it can only add some constant labels or information from a user's request, e.g., path, URL.

Maybe it is not a good idea to add status code to metric labels by default because of increasing metrics cardinality. I thought about adding an option to configure it somehow.

So, there are two questions left regarding all this:

1. Is it ok to add status code to metrics? I think it is if you want to use the RED method and differentiate E's.
2. Looking through the code, it is not clear how to fulfill this request, but I hope that someone can give me a hint.

[nabokihms]

Closed by #3182