Hardware sources of entropy on an FPGA
Metastability is really not a viable option in a modern FPGA technology because the metastable timing window is tiny - many orders of magnitude smaller than the setup/hold timing uncertainty window, which is dominated by things like clock skew, routing delays and variations with voltage and temperature. 

Unfortunately there's a lot of confusion about metastability and this timing window is sometimes called the metastable timing window, as other (sometimes unexpected) sources of uncertainty in an output due to clock timings are loosely lumped together and incorrectly called metastability. 

While this wider window generates uncertainty in the output, it's highly correlated with the above causes, not entropic (outside the tiny true metastable window). 

If you need the mathematical and practical details, search the Usenet comp.arch.fpga newsgroup for "metastability" and "Peter Alfke". 

TL/DR : look elsewhere for entropy : an avalanche diode, (i.e. a zener well above 6V, say 12V) amplified, sliced to logic levels, fed to an input pin would be one good choice.