pmrep conf: use newly added derived metric feature in proc metrics

Since scaling is requested we need to ignore the incompatible
case of novalue() which would otherwise cause an error.

src/pmrep/conf/proc.conf

@@ -42,6 +42,7 @@ instinfo = no
 unitinfo = no
 precision = 2
 repeat_header = auto
+ignore_incompat = yes
 proc.psinfo.pid   = PID,,,,7
 proc.psinfo.cmd   = Command,,,,12
 cpupct            = proc.psinfo.cpu.pct
@@ -77,7 +78,12 @@ iotot.unit        = KB/s
 iotot.width       = 8
 net               = bpf.proc.net.total.bytes
 net.label         = Net KB/s
-net.formula       = bpf.proc.net.tcp.recv.bytes + bpf.proc.net.tcp.send.bytes + bpf.proc.net.udp.recv.bytes + bpf.proc.net.udp.send.bytes
+net.formula       =
+  defined(bpf.proc.net.tcp.recv.bytes)
+  ?
+    bpf.proc.net.tcp.recv.bytes + bpf.proc.net.tcp.send.bytes + bpf.proc.net.udp.recv.bytes + bpf.proc.net.udp.send.bytes
+  :
+    novalue()
 net.unit          = KB/s
 net.width         = 8