CMOS pulse-width converter
Blue is pulsed input signal, green is Cap voltage.


Edit:
Regarding to one pulse only reset the 2pF Cap imediately after input pulse go low is undesirable so disconnect the Cap rather and reset is before next measurement only. For holding the voltage for longer time buffer it to bigger Cap. You can disconnect the bigger one after input signal goes low (+ charging time of big one) so it isnt affect by discharging the small one.