Again, why? There just isn't a demand for consumers to measure their light bulb socket voltage, so such a product would be niche, which tends to mean more expensive and difficult to find than getting this done in more conventional ways, whether "getting this done" means a voltage reading or instead solving the core problem.
What is a voltage critical appls? Why do you need this on an extension cord at all and then, why a bulb socket instead of just a regular extension cord that any typical outlet voltage monitor plugs into?
Monitoring voltage remotely like this, the extension cord doesn't even need to have any significant size (low gauge), any cheap thin one would work, so the cost could end up as low to not try to buy a niche product that has a bulb socket or fiddle with adapting it.
More often than not, your voltage drop is going to be between an outlet and the powered device, not showing up on something plugged into a separate outlet/hole, unless you think your wiring is substandard for the circuit's current rating and in that case, don't hesitate to fix the wiring.
If you are observing that your site suffers brownouts, more info might help, what problem is the reduced voltage going to cause and why not use a line conditioner to get over that?