Logging measurements from an analog Gas Meter
Going off John U's comment above, you could consider a linear light sensor array, such as this:

http://parallax.com/product/28317

It has a lens, and is focusable.  It has 128x1 pixels, and can be read with an arduino.  Set it up to look at the graticule on the last digit, and use an arduino to observe the white/dark stripes as they move across the linescan sensor.  It's sensitive to infrared light, so you can light it at night without attracting unwanted attention.

Use the arduino to turn that into a value.

Another similar option would be to tear apart an optical mouse and use its sensor to watch the numbers move below it.  Might be too far to sense it without changing the lens, though.

Also, consider attaching a microphone or sensitive accelerometer to the bellows casing itself, near the part which switches each time a bellow fills.  You might be able to sense that "click" well enough to discern it from outside noise.