What is the end product of nitrate metabolism?

There is no end product of nitrate metabolism. Nitrates, themselves, are one of the end products of protein metabolism.

Nitrite resulting from nitrate might confuse some people into thinking that there is nitrate metabolism. In the case of a UTI caused by bacteria, the bacteria produce an enzyme that changes urinary nitrates to nitrites, but this is not metabolism. It's the result of an enzymatic action.