I have one Yuncore AP with OpenWRT connected via PoE to a switch. I can get the power and current usage consulting Power Status table in the switch, but I need obtain these values on the AP side, and I couldn't find a way to get them in OpenWRT.
In general it is unlikely this information will be available from the client side.
The client hardware (POE injector stuff, even if integrated into the device) is usually not sophisticated, and won't have telemetry.
The client hardware can't measure line losses, which may be a significant amount of power depending on the length of the cable, power draw, and voltage. Even if it did have a number, it wouldn't be very accurate.