Of all the US Galaxy S4 variants, it's a safe bet that the Metro PCS version is one of the lesser-owned models. That said, if you count yourself among the ranks of SGH-M919N owners, your Android 4.3 update is ready. The official changelog doesn't look any different from the comparable OTAs for the other variants, but it's always nice to get a big firmware update like this nonetheless.
If you're curious what OS changes Android 4.3 brought with it, you can check out our overview. The new firmware version is M919NUVUEMK5 and, like most updates for Android devices these days, is likely rolling out in stages.