I did a project with a french Linky a few years ago that used an ESP8266 to grab the serial data stream from that exposed port on the bottom and log the interesting line items it prints out to a InfluxDB timeseries database running on a local server and use Grafana to get a dashboard of various metrics.

I remember having to do some sleuthing with the serial port as it wasn't something standard - it was like 7 bits, no parity, 1 stop bit, or something like that. And a small circuit to do level shifting for the ESP8266.

The result was data on wattage draw approx every 1 second. This allowed for some very, very fine analysis in the data.

I tell people about this when I talk about various sidechannel attacks on things that use data like this- for instance, after observing the data for a while, I could tell which floor of the house someone was on by watching transients of the wattage and the quantized changes of various devices turned on and off - e.g. the kitchen has a kettle that uses 1220 +/- 5 watts, every time, and nothing else in the house results in a step change of that value. The top floor bedroom has lights that use exactly 42 watts. The laundry machine starts up with a very specific ramp and then oscillates between two levels of power with a period of 30 seconds, etc.

You can figure a lot out about the movements of people and what they are doing by simply having fine enough time resolution on a single number - the total power consumption of the entire house.