Full disclosure - everything factual here I'm about to write comes from the same paper that AlexP quoted in the comments, I'm just going to go over a bit more things. The paper is here if you want to look at it.
Metallic Hydrogen is metastable
Once created, as long as it's kept in the same conditions as it was created in, it's expected to be metastable. However, this may prove tricky, as it requires around 500 GPa to create, or about 5,000,000 atmospheres of pressure. As in, not that keeping it is tricky but making the stuff is. It's unclear if we've actually created the stuff, by the way, a lab claimed that they made it, however, due to the nature of the small sample and the fact that it was between diamond anvils, they were unable to perform conclusive tests. But, given the insane difficulty in making the stuff, there is what to believe that it's going to be metastable at the pressure it was created in.
And, as the paper states, dropping the pressure even to about 10 GPa, or a mere 100,000 atmospheres, is about the least you can do without the metallic hydrogen decaying. To give you another comparison, this is roughly the pressure that diamonds form at. I wouldn't actually worry about this part, though. Metallic hydrogen, at least by our current means, is impossible to mass produce. If we develop the means to mass produce it, that means we've develop specialized equipment to handle creating something at that kind of pressure, so it's very conceivable that we'll have equivalent level fuel tanks to hold it. (If you're wondering, diamond fuel tanks can easily withstand that much pressure, but that's currently impractical, for various reasons.)