Let’s say metallic hydrogen becomes an economically viable resource that we are able to either make ourselves or mine from a gas giant, how would we use it as a power source for vehicles both planetary and space based?
If metallic hydrogen is metastable at STP (or at least pressures that can be realistically engineered by humans) then all you have to do is to warm is up to about 1000K and it'll go foom vigorously as it turns into a gas, which you can then blow out of a conventional rocket nozzle. You could make a single-stage-to-orbit rocket without the need to muck about with beamed power or hazardous nuclear engines because so much energy will be released by this process. You could, in theory, use the same stuff as a heat source in a gas turbine or similar, providing thrust or driving a generator as necessary. It would also make a brilliantly dense fuel storage medium for other hydrogen-driven engines (fusion rockets, for example) where conventional cryogenic hydrogen tanks might be rather inconvenient.
The really interesting stuff might actually be if it turns out to be a room-temperature superconductor too, which might actually be more valuable than a super rocket fuel. It is also possible that it might make a very good charged-particle radiation shield, if it has more hydrogen nuclei per unit volume than alternatives like polyethylene (its density is lower than polyethylene, but there are no carbon atoms, but I'm not going to work out interaction cross-sections). And assuming that being hit by cosmic radiation doesn't make it explosively sublime. Maybe its best not to risk that one...
If it isn't metastable (and research suggests that even if it remains metal at STP it'll spontaneously break down due to quantum tunelling, which would make it quite a dangerous material to work with), then you're faced with the problem of making a storage tank capable of withstanding 25Gpa (or about 250000 atmospheres) of pressure. This is likely to be so big and heavy and awkward and expensive that it simply won't be worth the effort and will remain merely an item of scientific interest.
Probably you'll have to wait til someone actually manages to make macroscopic quantities of the stuff to find out. Mining it from gas giants is likely to be forever implausible, due to those punishly deep gravity wells.