You can set up iptables to manage the counters - they can even be made to survive reboots with a save/restore or manually clearing/setting the counters to specific values.
If you don't already have iptables rules you just need to add at least one rule to the input and output chains that allows everything for example and it will provide what you want:
iptables -A INPUT -j ACCEPT
iptables -A OUTPUT -j ACCEPT
Then you can see the totals:
root@devcloner:~# iptables -n -vL
Chain INPUT (policy ACCEPT 2850K packets, 4183M bytes)
pkts bytes target prot opt in out source destination
22M 32G ACCEPT all -- * * 0.0.0.0/0 0.0.0.0/0
Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain OUTPUT (policy ACCEPT 657K packets, 43M bytes)
pkts bytes target prot opt in out source destination
12951 813K ACCEPT all -- * * 0.0.0.0/0 0.0.0.0/0
-x will show the full byte counter:
root@devcloner:~# iptables -n -vL -x
Chain INPUT (policy ACCEPT 2850263 packets, 4182667884 bytes)
pkts bytes target prot opt in out source destination
22285352 32724735127 ACCEPT all -- * * 0.0.0.0/0 0.0.0.0/0
Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
pkts bytes target prot opt in out source destination
Chain OUTPUT (policy ACCEPT 657099 packets, 43320848 bytes)
pkts bytes target prot opt in out source destination
102453 6738544 ACCEPT all -- * * 0.0.0.0/0 0.0.0.0/0
That info is probably parseable somewhere from /proc or /sys too.