middleware/metrics: export actual size (#448)

The `*_size*` metrics now export the actual packet length, not the
advertised one (although that might be nice as well).

request/request.go

@@ -106,7 +106,10 @@ func (r *Request) Do() bool {
 	return false
 }
 
-// Size returns if UDP buffer size advertised in the requests OPT record.
+// Len returns the length in bytes in the request.
+func (r *Request) Len() int { return r.Req.Len() }
+
+// Size returns if buffer size *advertised* in the requests OPT record.
 // Or when the request was over TCP, we return the maximum allowed size of 64K.
 func (r *Request) Size() int {
 	if r.size != 0 {