Remove (or use) network stats that are never incremented

**Description**

Some stats can never be incremented, they should be removed or incremented where possible.

**Do you have a specific solution in mind?**

An example of such stat is ICMP.V{4,6}.PacketsSent.Echo. It is never incremented. In `transport/icmp/endpoint.go`, it should be incremented when an outgoing Echo packet is sent.