feature: measure TCP connection backlog

[federicotdn]

I was looking into Beyla to see if it is possible to use it to measure TCP connection backlogs - with this I mean the number of incoming connections to a TCP listening socket that have not been accept()'ed yet. Since this feature does not exist yet in Beyla, I propose it as a feature. This could probably be exported as a Gauge metric.

Some reference conversation in StackOverflow: https://stackoverflow.com/questions/4253454/question-about-listening-and-backlog-for-sockets/.