This page describes basically what I'm planning to do for the gas meter:

http://sustburbia.blogspot.com/2009/11/using-arduino-to-monitor-gas.html

...which gives a strong pulse every 10dm³ of gas consumed (and nine weaker ones for the in-between digits).

The method is not too dissimilar for the electric meter (which has a flashing LED for every 1Wh consumed).

I might use an Arduino instead of RPi (if I can get logging to SD card working easily) because it offers analogue inputs so I can easily add e.g. temperature sensors later, and maybe do all the optical sensing in software instead of needing a couple of opamps. This page looks useful: 

https://howtomechatronics.com/tutorials/arduino/arduino-sd-card-data-logging-excel-tutorial/

We'll see...