I am keen on getting a Carl Zeiss Jena DDR Electric MC S 135mm f/3.5 lens but i'm wondering if this lens will meter on a DSLR, specifically Nikon D7100?
This is, as far as any other camera than an "electric" compatible Pentacon/Praktica (eg the Praktica LLC) is concerned, a fully manual/stop-down-metering M42 lens. The automatic aperture (for a bright viewfinder. Not shutter priority!) will only ever work on a native M42 camera, and the meter-coupling facility of the "electric" lenses will only work with said very specific M42 cameras.
Note that M42 lenses and Nikon F cameras are a particularly awkward (read: unserviceable if you look at it realistically...) combination. You'll be either limited to macro range or introducing additional glass.
Note that you might need an M42 adapter that depresses the automatic-aperture control pin on the back of the lens, some "electric" (and other "automatic" M42) lenses cannot be made to stop down at all without that (unless you modify the lens).
Generally, such lenses will work on any camera that allows metering without a known lens attached. Aperture simulation is not needed since you will be metering at working aperture, so what you get in the viewfinder/meter is what you get on the sensor. Consumer DSLRs are notoriously often crippled regarding that capability.
The D7100 (unlike, eg, the D3100) is known to be able to meter with "CPU-less" mechanical Nikon F mount lenses; since an adapter will usually pretend to be such a lens, it can be expected to work (obviously in aperture priority and manual only, no EXIF).
BTW, the "electric" contacts on various M42 lenses (Praktica/Pentacon system, Zenit system...) just couple to simple potentiometers or switched resistor networks, not to a CPU or ROM....
Theoretically your camera can meter with this lens. The "electric" in the name of the lens means that it is made to have an electrical connection with the camera. Your camera can then know at what aperture the lens is set, and meter correctly. You do need a lens mount that allows a proper electric connection, and the data for the lens may need to be 'translated' for the camera.
If a lens has a mechanical coupling for the aperture, the camera can also know which aperture is used, and expose correctly. The camera only needs to know what apertures the lens is capable of. Higher end Nikons (like the D7xxx series) have a 'non AI lens list' where you can set this.
So the answer is, yes it is possible, but you need a lens mount that allows it.