Simple Metronome Circuit w/ Linear Pot Control
You would be better using a microcontroller with a crystal for this, since the accuracy would be much much better. However, here is a (very rough) 555 based circuit I threw together, that uses a constant current source to charge the cap - it's very rough but does approximately 100mV/Hz from 9V downwards (e.g. 8.99V = 0.1Hz, 8.9V = 1Hz, 7V = 20Hz)
You can tweak it to scale the V/Hz as desired (and also tweak the cap value and pulse time (remember this doesn't change so ideally is short to stop the error becoming large as the frequency increases), generally tidy things up a bit) A rail to rail (ideally precision) opamp would be needed. 

Circuit:

  

Simulation of the voltage control dropping and frequency scaling linearly:



Simulation set for 9 - 1.7V = 17Hz (notice error of ~0.4Hz from pulse width):



Improved Version

Here's an improved version which uses a Wilson Current Mirror and a smaller capacitance, so a precision capacitor and trim cap can be used. I set it up for 0.45V/Hz, so with 9V and the 10 turn 10k&Omega; pot you will get 2Hz per turn (up to 9V = 20Hz in theory - in practice the current source compliance will limit things to around 17Hz, which is okay, but the pot cannot be fully turned all the way to 9V or the current source will "drop out")
So again, as it's a "rush job" things will need tweaking a bit to suit.
The large linear range is difficult to handle with one pot, so the slower range will be harder to set - it may be worth having another range resistor, to switch in for the lower range (e.g. to set it up for 0.45V/10mHz or similar)  

Anyway, here it is (pot upper/lower represent the pot):