If you want to drive the meter from a PWM output from a micro, if you go above about 1kHz (maybe 100 Hz) then the response time of the meter will be sufficient that you don't need any damping. With a 5 V output, a 49k resistor in series will suffice and the the meter will read 0 for 0% duty cycle, 5 for 50% duty cycle, 10 for 100% duty cycle etc.
If you want to damp the signal with a capacitor, split the 49k resistor into two approximately equal series resistors, and put a capacitor with its positive connected to where the two resistors join, and its negative to ground. A 100 uF will give you a time constant of around 1 second. If you split the resistors unevenly, you need to use a bigger capacitor for the same timing constant.
Do not use a diode. It will give you the peak, not the average voltage.