RPM measurement for telemetry

Is it possible to measure rpm for telemetry only without the governor ?
And if so, does it work on pixhawk only or other controllers as well ?

Yes, you can measure RPM without using the Governor. What mode of the rotor speed controller are you currently using?

I believe it will work on other controllers. What are you looiking to use it on? Here is the wiki page on RPM sensor setup.

I have a Hobbywing Platinum V3 ESC which has a governor built in
and also has a RPM output.
It would be nice to have RPM sent via telemetry.
The flight controller is an Omnibus F4 V3.
So in my understanding from the wiki the RPM output from the ESC can be connected to PWM6 (the highest PWM output) and
RPM_SCALING = 1/6 (6 pole motor)