Unless there's a deliberate vulnerability in Metasploitable regarding password storage, this is impossible. In a system that isn't horribly insecure, the password is not stored as such: what is stored is a hash of the password. The only way to find the password given the hash is to try calculating the hash of a guess and check whether the hash of the guess is equal to the hash stored in the password database.
Not only that, but a password hashing function is deliberately made slow, so that brute-force guessing can't be done quickly.
There are tools to crack passwords, such as John the Ripper, but the reason they're slow is that password cracking is designed to be intrinsically slow. It can still be a simple command, but if the password isn't extremely weak then the command will have to run for a very long time, and if the password is good and the system isn't horribly broken then the command should not be able to find the password during your lifetime. After all, what would be the point of using a password that can be cracked?