Getting Network Statistics

I have a subscription based Oracle database that is being converted
to Postgres under Redhat Linux 8.0. Usage is charged based on a
combination of CPU used and the number of bytes sent.

For Postgres I can see how to get CPU usage for the query/connection
from parsing the log for a connect statement, getting the PID and either
getting the CPU from the Postgres log or using the PID to find the Linux
file /proc/xxx/stats (where xxx ix the PID) and parsing the stats file
for the user and system CPU.

I still need to find the number of bytes transmitted to the client.
Oracle provides a "virtual" table, V$SESSTAT that provides that
information, but I have no idea how to get that information from
Postgres, or if that is not possible, from the operating system.

Any suggestions are much appreciated.

Thanks,

Jeff