Current spans map: consider using RemoteAddr field

[mariomac]

During the aggregation of HTTP call spans, the eBPF code stores ongoing spans by using the request context pointer as key (private field ctx in type http.Request).

While this is performant (only have to store a pointer as map key), this is an implementation detail that might be broken if the http.Request implementation changes.

We could consider using something more stable, such as the public field http.Request.RemoteAddr. However this might involve some extra processing because this field is a variable-length string.

[mariomac]

There is something similar already done. Closing by now.